ChatGPT answers, unedited

If your key requirement is one agency managing multiple clients while keeping each client’s sending reputation, domains, lists, and reporting isolated, I’d focus less on the email editor and more on the platform’s account architecture + deliverability controls.

My shortlist

Campaign Monitor is particularly designed around agencies: its agency offering lets you manage separate client accounts while keeping each client's work organized, and it specifically highlights separate client accounts and sending needs. Campaign Monitor Hack'celeration

The important part: don't put all clients in one sending pool

If by "separate sending reputations" you mean you don't want Client A's poor engagement/spam complaints to hurt Client B, I'd structure it like this:

Agency
→ Client A account → send.clientA.com
→ Client B account → send.clientB.com
→ Client C account → send.clientC.com

Ideally each client has:

• Their own platform account/sub-account
• Their own sending domain or sending subdomain
• Their own SPF/DKIM/DMARC configuration
• Their own suppression/bounce history
• Their own engagement data
• Their own reputation monitoring
• Separate reporting

This is more important than whether the platform claims to have an "agency dashboard." For example, Klaviyo explicitly supports branded sending domains and explains that separate sending domains can establish distinct sender reputations for different types of traffic. Klaviyo Help Center Klaviyo Help Center

What I'd choose based on your agency model

If you have 5–50 ordinary SMB clients:
I'd look closely at Campaign Monitor or Brevo. Campaign Monitor has particularly explicit agency/client-account infrastructure. Campaign Monitor

If you're selling sophisticated automation:
I'd look at ActiveCampaign. Its automation capabilities are substantially deeper, and its documentation recommends using subdomains to segment sending and help manage domain reputation. ActiveCampaign Help Center

If your clients are Shopify/e-commerce brands:
I'd strongly consider Klaviyo, with a separate Klaviyo account for each client. Its account/domain model is particularly well suited to keeping each brand's email ecosystem distinct. Klaviyo Help Center

If you have dozens of small clients and care heavily about margin:
Brevo is worth investigating because its agency model and send-based pricing can make the economics different from per-contact platforms. Hack'celeration

One thing I'd avoid

I would not create one giant account containing:

Client A list + Client B list + Client C list → same sending infrastructure

even if you can technically segment the audiences.

It makes client isolation, offboarding, permissions, billing, troubleshooting, and especially deliverability substantially messier. Current agency-focused comparisons similarly emphasize separate client accounts, domains, lists, and reputation as core requirements. Mailercloud SendGrove

If you tell me roughly how many clients you manage (e.g. 10, 50, 200), monthly email volume per client, and whether they're mostly B2B or e-commerce, I can narrow this down to the architecture I'd use and compare the likely cost.

If your key requirement is multiple client accounts with genuinely separate sending reputations, I’d focus less on the email editor and more on the platform’s account/sub-account architecture, domain authentication, IP strategy, permissions, and deliverability controls.

The platforms I’d look at

Campaign Monitor BestSendsCampaign Monitor is particularly interesting for your exact requirement: it explicitly supports separate client accounts, with each client's subscribers, campaigns, templates, analytics and sending settings kept separate. It also has agency-oriented permissions and private-label capabilities. Campaign Monitor

The important distinction: domain vs. platform account

If by "separate sending reputations" you mean:

Client A has a bad campaign → Client B's deliverability shouldn't suffer.

Then I would not put all clients on one shared sending identity.

Ideally, each client should have:

• Their own sending domain/subdomain
• Their own SPF/DKIM/DMARC configuration
• Their own subscriber database and suppression data
• Their own sending history/reputation
• Ideally their own dedicated IP strategy if their volume warrants it

That creates much better blast-radius isolation between clients. US Tech Automations

For example:

Your Agency
│
├── Client A
│   ├── account/workspace
│   ├── mail.clientA.com
│   ├── Client A lists
│   └── Client A reputation
│
├── Client B
│   ├── account/workspace
│   ├── mail.clientB.com
│   ├── Client B lists
│   └── Client B reputation
│
└── Client C
    ├── account/workspace
    ├── mail.clientC.com
    ├── Client C lists
    └── Client C reputation

What I'd choose based on your agency model

If you're primarily a traditional marketing agency:
I'd investigate Campaign Monitor first. Its agency architecture is unusually aligned with managing independent client brands from one administrative environment. Campaign Monitor

If you're selling sophisticated lifecycle/automation work:
I'd look closely at ActiveCampaign. Its agency/partner tooling plus deeper automation and CRM capabilities make it more suitable when the actual service you're selling is complex lifecycle automation. BestSends

If most clients are Shopify/e-commerce:
I'd look at Klaviyo, where having separate client accounts is natural and the platform is heavily oriented around e-commerce lifecycle marketing. BestSends

If you have a lot of small clients:
Brevo is worth considering because its send-based economics can work better when you have many relatively small databases. BestSends

One thing I'd avoid

I wouldn't choose a platform simply because it offers an "agency dashboard." Multi-account access is not the same thing as deliverability isolation.

Before signing up, I'd specifically ask the vendor:

1. Can every client have a completely separate sending domain?
2. Are client accounts logically isolated?
3. Are IP pools shared between clients?
4. Can a client use a dedicated IP?
5. Can I manage all clients from one agency login?
6. Can clients have restricted access to only their account?
7. Can billing/reporting be separated by client?
8. If Client A gets a high complaint rate, what exactly can affect Client B?
9. Can clients bring their own domain and authentication records?
10. Can I export/transfer a client's lists and automations if they leave the agency?

For your specific requirement, I'd put Campaign Monitor and ActiveCampaign at the top of the initial evaluation, then Klaviyo if you're primarily e-commerce. The deciding factor should be how much isolation you get at the sending infrastructure level, not just how conveniently you can switch between client dashboards.

What I'd choose based on your agency model

If you're primarily a traditional marketing agency:
I'd investigate Campaign Monitor first. Its agency architecture is unusually aligned with managing independent client brands from one administrative environment. Campaign Monitor

If you're selling sophisticated lifecycle/automation work:
I'd look closely at ActiveCampaign. Its agency/partner tooling plus deeper automation and CRM capabilities make it more suitable when the actual service you're selling is complex lifecycle automation. BestSends

If most clients are Shopify/e-commerce:
I'd look at Klaviyo, where having separate client accounts is natural and the platform is heavily oriented around e-commerce lifecycle marketing. BestSends

If you have a lot of small clients:
Brevo is worth considering because its send-based economics can work better when you have many relatively small databases.

One thing I'd avoid

I wouldn't choose a platform simply because it offers an "agency dashboard." Multi-account access is not the same thing as deliverability isolation.

If your agency manages multiple clients and you want each client’s email reputation isolated, I’d focus less on a traditional “email marketing platform” and more on the sending infrastructure underneath it.

My shortlist

1. Mailgun — probably the most natural fit

Mailgun has subaccounts specifically designed for this kind of setup. A primary agency account can create client subaccounts with separate sending domains, API keys, users, and IPs. You can also assign dedicated IP pools to individual subaccounts and report on activity by client. Mailgun Documentation Mailgun Documentation

That's a very good architecture for:

Agency → Client A subaccount → Client A domains/IPs
Agency → Client B subaccount → Client B domains/IPs
Agency → Client C subaccount → Client C domains/IPs

Mailgun also separates sending queues by domain, which can provide additional isolation. Mailgun Documentation

I'd look at Mailgun first if your agency wants centralized administration without building a lot of infrastructure.

2. Amazon SES — strongest isolation/control

Amazon Web Services's Amazon Simple Email Service has become particularly interesting for agencies because SES now has tenant management.

You can create a tenant for each client, with separate identities, configuration sets, templates and reputation metrics. AWS explicitly describes this as useful for service providers that need to isolate email reputation by client. AWS Documentation

You can go further with dedicated IP pools. AWS says dedicated IPs can isolate reputation between different components/tenants, while shared IPs cannot provide that isolation. AWS Documentation

The tradeoff is that SES is infrastructure rather than a polished marketing UI. You'd generally pair it with something like an email campaign application, your own software, or a higher-level marketing platform.

3. SendGrid — mature middle ground

Twilio SendGrid supports dedicated IPs and IP pools, and its infrastructure can isolate marketing and transactional traffic. Twilio

It's a reasonable choice if you want a mature email platform with a substantial UI/API ecosystem without going as deep into AWS as SES.

One important distinction

If by "separate sending reputations" you mean “Client A gets a bad complaint rate and I don't want it damaging Client B”, don't just create separate lists/campaigns inside one account.

You want isolation at the sending infrastructure/account/tenant level, ideally:

• Separate client sending domains
• Separate DKIM/SPF/DMARC configuration
• Separate subaccounts/tenants
• Separate suppression/bounce data where possible
• Separate IP pools when volume justifies dedicated IPs
• Separate reporting and credentials
• Ideally separate transactional vs. marketing streams

AWS's current tenant system is particularly explicit about preventing one tenant's reputation problems from affecting another. AWS Documentation

Also, don't automatically put every client on a dedicated IP. Both AWS and Mailgun note that dedicated IPs make the most sense at sufficient, consistent sending volume; low-volume senders can actually be better served by reputable shared infrastructure. AWS Documentation Mailgun Documentation

What I'd choose

For an agency, I'd narrow it to:

Mailgun → easiest combination of agency → subaccounts → client-level domains/IPs/reputation.

Amazon SES → best if you're comfortable being more technical and want maximum tenant/reputation control at scale.

SendGrid → good alternative if you want a more established general-purpose email platform.

If you tell me roughly how many clients you manage, emails/client/month, and whether you need a visual campaign builder, I can compare the three architectures and costs more specifically.

You can create a tenant for each client, with separate identities, configuration sets, templates and reputation metrics. AWS explicitly describes this as useful for service providers that need to isolate email reputation by client. AWS Documentation

You can go further with dedicated IP pools. AWS says dedicated IPs can isolate reputation between different components/tenants, while shared IPs cannot provide that isolation. AWS Documentation

The tradeoff is that SES is infrastructure rather than a polished marketing UI. You'd generally pair it with something like an email campaign application, your own software, or a higher-level marketing platform.

3. SendGrid — mature middle ground

Twilio SendGrid supports dedicated IPs and IP pools, and its infrastructure can isolate marketing and transactional traffic. twilio.com

It's a reasonable choice if you want a mature email platform with a substantial UI/API ecosystem without going as deep into AWS as SES.

One important distinction

If by "separate sending reputations" you mean “Client A gets a bad complaint rate and I don't want it damaging Client B”, don't just create separate lists/campaigns inside one account.

You want isolation at the sending infrastructure/account/tenant level, ideally:

Yes — if your main requirement is keeping each client’s sending reputation isolated, I would prioritize platforms with true client-level accounts/sub-accounts, not just separate lists inside one account.

My shortlist for an agency in 2026:

My #1: ActiveCampaign

For a typical marketing agency, I'd probably choose ActiveCampaign.

Its agency program is specifically designed around managing/reselling separate client accounts, rather than putting every client into one giant account. That matters because you can give each client their own contacts, automations, domains, permissions and sending infrastructure. Recent agency comparisons also consistently put it near the top for multi-account management and automation. BestSends PG Systems Lab

The architecture I'd want is:

Agency
→ Client A account → clientA.com sending domain
→ Client B account → clientB.com sending domain
→ Client C account → clientC.com sending domain

rather than:

One agency account
→ Client A list
→ Client B list
→ Client C list

The second arrangement makes separation much harder.

Campaign Monitor is especially interesting

Campaign Monitor may actually be the better choice if your agency is primarily doing newsletters/campaigns rather than sophisticated CRM automation.

Its agency/client-management model is specifically built around isolated client accounts, with separate sending domains and client-level management. Hack'celeration

So I'd lean:

• ActiveCampaign → lifecycle marketing, lead nurturing, CRM, sophisticated automations
• Campaign Monitor → newsletters, campaigns, creative/design-heavy agency work
• Klaviyo → ecommerce clients
• Brevo → lots of smaller/price-sensitive clients

One important distinction about "separate reputation"

Don't assume that separate accounts automatically means completely separate deliverability.

For each client, I'd configure:

• Their own sending domain/subdomain
• SPF
• DKIM
• DMARC
• Their own From address
• Appropriate bounce/complaint handling
• Separate suppression/unsubscribe data
• Ideally separate dedicated IP infrastructure only when the client's volume justifies it

The sending domain is particularly important. You don't want Client A's poor list hygiene or spam complaints damaging Client B's domain reputation. Agency-focused comparisons specifically identify client/domain isolation as a major consideration. US Tech Automations US Tech Automations

If you're building an agency from scratch

I'd probably standardize on ActiveCampaign + one account per client and make domain authentication part of your onboarding checklist.

If you tell me roughly how many clients you manage (e.g. 5, 20, 100), monthly email volume, and whether they're mostly B2B or ecommerce, I can narrow this down to the best 2–3 options and compare agency pricing, sub-accounts, white-labeling, dedicated IPs, and deliverability isolation.

The second arrangement makes separation much harder.

Campaign Monitor is especially interesting

Campaign Monitor may actually be the better choice if your agency is primarily doing newsletters/campaigns rather than sophisticated CRM automation.

For an agency managing multiple clients, I would prioritize client isolation, separate sending reputations, permissions, reporting, and handoff ability over the usual “best email marketing platform” rankings.

The strongest options depend on your agency model:

Campaign Monitor BestSends## My shortlist by agency type

1. You manage 5–50 clients and want one operating system

Pick: ActiveCampaign

Good fit if you sell:

• email strategy retainers
• automation builds
• lead nurturing
• CRM integrations
• customer journeys

Each client should have:

• their own account/workspace
• their own sending domains
• their own DKIM/SPF/DMARC setup
• their own suppression lists

That prevents one client's poor list hygiene from damaging another client's deliverability. BestSends

2. You mostly run newsletters and campaigns

Pick: Campaign Monitor

It is more agency-oriented out of the box:

• separate client accounts
• client permissions
• reporting
• easier client handoff

Campaign Monitor specifically positions its agency offering around managing separate client accounts and keeping work separated. Campaign Monitor

3. You have lots of small clients

Pick: Brevo

Example:

• 30 restaurants
• 20 local service businesses
• monthly newsletters
• basic automations

The economics can work better because you are not paying premium automation-platform pricing for every small account. BestSends

Architecture I would use for an agency

I would avoid putting every client into one master sending account.

A cleaner setup:

Agency Admin
│
├── Client A
│   ├── Sending domain: mail.clientA.com
│   ├── Lists
│   ├── Automations
│   └── Reporting
│
├── Client B
│   ├── Sending domain: mail.clientB.com
│   ├── Lists
│   ├── Automations
│   └── Reporting
│
└── Client C
    ├── Sending domain: mail.clientC.com
    ├── Lists
    ├── Automations
    └── Reporting

Avoid these setups

• ❌ One agency domain sending for every client
• ❌ One shared list database
• ❌ One IP/domain reputation shared across unrelated clients
• ❌ Client-owned accounts that your agency cannot export or transition

My recommendation

For most email marketing agencies:

1. ActiveCampaign → best overall if automation is your service
2. Campaign Monitor → best if you manage many newsletter clients
3. Brevo → best margin-friendly choice for many small accounts
4. HubSpot → best if clients are B2B and sales-driven

If you tell me:

• number of clients (5, 20, 100+)
• B2B vs ecommerce vs newsletters
• approximate monthly sends
• whether clients need login access

…I can narrow this to a specific stack and account structure.

1. ActiveCampaign → best overall if automation is your service
2. Campaign Monitor → best if you manage many newsletter clients
3. Brevo → best margin-friendly choice for many small accounts
4. HubSpot → best if clients are B2B and sales-driven

If you tell me:

• number of clients (5, 20, 100+)
• B2B vs ecommerce vs newsletters
• approximate monthly sends
• whether clients need login access

…I can narrow this to a specific stack and account structure.

If your priority is one agency dashboard + multiple client accounts + genuinely separate sending reputations, I’d narrow it to a few architectures rather than the usual “best email marketing platform” list.

My picks

🥇 My recommendation: Mailgun + an agency-facing marketing layer

If sending reputation isolation is the most important requirement, I would seriously consider Mailgun as the underlying sending infrastructure.

Mailgun supports actual subaccounts, where each subaccount can have its own sending domains, IPs, API keys and users. Usage can still roll up to the agency's primary account. Mailgun Developer Documentation

It also separates sending queues by domain, and offers dedicated IPs/pools for sufficiently high-volume senders. Mailgun Developer Documentation

That gives you an architecture like:

YOUR AGENCY
│
├── Client A
│   ├── clientA.com / mail.clientA.com
│   ├── Client A Mailgun subaccount
│   └── Client A reputation
│
├── Client B
│   ├── clientB.com / mail.clientB.com
│   ├── Client B Mailgun subaccount
│   └── Client B reputation
│
└── Client C
    ├── clientC.com / mail.clientC.com
    ├── Client C Mailgun subaccount
    └── Client C reputation

That's much safer than putting 20 clients into one ESP account and assuming that “separate lists” means “separate reputation.”

If you want everything in one marketing platform

ActiveCampaign is probably my first choice. It is stronger than Mailgun as an actual marketing application—automations, CRM, segmentation, campaigns, etc.—and has agency-oriented multi-account capabilities. Current agency comparisons also rank it particularly highly for centralized client management and automation. BestSends Sasanova

The caveat is that I'd still pay close attention to how each client's sending domain and IP infrastructure is configured. A sub-account alone doesn't necessarily mean complete IP-level isolation.

If you're a lead-gen/cold-email agency

That's a different recommendation.

I'd look at an agency-first platform such as SendKit. Its current agency offering explicitly uses separate workspaces with dedicated IP infrastructure, meaning one client's sending reputation isn't supposed to bleed into another's. It also offers white-label functionality. SendKit

For cold outbound, I'd actually prefer this kind of architecture over traditional Mailchimp/ActiveCampaign-style newsletter software.

One important distinction

There are two different reputations you should isolate:

1. Domain reputation — clientA.com vs clientB.com
2. IP reputation — Client A's sending IP vs Client B's

You absolutely want each client sending through their own authenticated domain/subdomain with their own SPF/DKIM/DMARC configuration. For higher-volume clients, dedicated IPs/pools can provide another layer of isolation. Customer.io and Mailgun both document the distinction between shared and dedicated IP infrastructure. Customer.io Mailgun Developer Documentation

So I would not choose a platform simply because it says “agency subaccounts.” I'd ask:

“If Client A gets a major spam complaint spike, exactly what infrastructure does Client B share with them?”

That's the question that exposes whether the isolation is real.

If you tell me roughly how many clients you manage (e.g. 5, 20, 100), monthly emails per client, and whether this is newsletters/marketing automation or cold email, I can give you a much more specific stack recommendation and pricing comparison.

YOUR AGENCY
│
├── Client A
│   ├── clientA.com / mail.clientA.com
│   ├── Client A Mailgun subaccount
│   └── Client A reputation
│
├── Client B
│   ├── clientB.com / mail.clientB.com
│   ├── Client B Mailgun subaccount
│   └── Client B reputation
│
└── Client C
    ├── clientC.com / mail.clientC.com
    ├── Client C Mailgun subaccount
    └── Client C reputation

That's much safer than putting 20 clients into one ESP account and assuming that “separate lists” means “separate reputation.”

If you want everything in one marketing platform

ActiveCampaign is probably my first choice. It is stronger than Mailgun as an actual marketing application—automations, CRM, segmentation, campaigns, etc.—and has agency-oriented multi-account capabilities. Current agency comparisons also rank it particularly highly for centralized client management and automation. BestSends Sasanova

The caveat is that I'd still pay close attention to how each client's sending domain and IP infrastructure is configured. A sub-account alone doesn't necessarily mean complete IP-level isolation.

If you're a lead-gen/cold-email agency

That's a different recommendation.

I'd look at an agency-first platform such as SendKit. Its current agency offering explicitly uses separate workspaces with dedicated IP infrastructure, meaning one client's sending reputation isn't supposed to bleed into another's. It also offers white-label functionality.

For cold outbound, I'd actually prefer this kind of architecture over traditional Mailchimp/ActiveCampaign-style newsletter software.

One important distinction

There are two different reputations you should isolate:

If your main requirement is agency-level management + genuinely separate sending reputations per client, I’d narrow it to Brevo, Mailgun, and ActiveCampaign—but they solve slightly different problems.

My ranking

🥇 Best fit: Brevo Enterprise

For what you're describing, Brevo Enterprise is probably the cleanest fit.

Brevo has an explicit sub-organization/sub-account architecture intended for organizations managing multiple entities. Each sub-account can have its own contacts, API key, users/permissions, and sending setup. More importantly for your use case, dedicated IPs can be assigned to individual sub-organizations. Brevo Help Brevo Help

So you could structure it like:

• Agency master account - Client A → mail.clientA.com → IP A
• Client B → mail.clientB.com → IP B
• Client C → mail.clientC.com → IP C
• Client D → mail.clientD.com → IP D

That gives you much better operational separation than simply creating multiple campaigns inside one account.

Brevo specifically says its Enterprise sub-account system is designed for agencies managing various clients and supports separate or common dedicated IPs for sub-accounts. Brevo Help

One caveat: dedicated IPs are primarily worthwhile at meaningful sending volumes. Brevo's current Enterprise documentation includes dedicated IP capability, but you still need to warm each IP properly. Brevo Help

🥈 Best if deliverability is your #1 priority: Mailgun

If you're less interested in a polished "marketing agency platform" and more interested in controlling email infrastructure, I'd seriously consider Mailgun.

Mailgun's subaccounts are unusually well designed for your use case. A subaccount can have its own:

• sending domains
• API keys
• users
• IP pools
• reporting
• event logs

The parent account can manage these centrally. Mailgun Documentation Mailgun Documentation

You can even assign different IP pools to different subaccounts. Mailgun Documentation

That makes this architecture possible:

Agency → Mailgun master → one subaccount/client → separate domains + IP pool per client

Mailgun also recommends reputation isolation with dedicated IPs for high-volume senders and separates sending queues by domain. Mailgun Documentation

I'd choose Mailgun over Brevo if your agency is technically sophisticated and cares more about deliverability/infrastructure than having a traditional email-marketing UI.

🥉 ActiveCampaign

I'd choose ActiveCampaign if the agency's product is really marketing automation, rather than simply sending newsletters.

It's strong for managing multiple client accounts and agency workflows, and current agency-focused comparisons consistently put it near the top for multi-account management and automation. BestSends

But there's an important distinction:

"Separate client account" ≠ "completely isolated sending reputation."

If your requirement is specifically:

"I don't want Client A's bad list or spam complaints affecting Client B."

I'd prioritize a platform where you can explicitly control subaccounts + sending domains + IP allocation, rather than choosing purely on CRM/automation features.

What I would do

For an agency managing, say, 10–50 clients, I'd probably use:

Brevo Enterprise if the clients are primarily traditional marketing/email campaigns.

Mailgun if you're building a more sophisticated email infrastructure/API layer.

ActiveCampaign if you're selling sophisticated CRM + automation as part of your agency service.

And I would not put all clients into one giant account with the same sending domain/IP simply because the platform lets you create separate lists. That's account-level organization, not necessarily reputation isolation.

One other important point: separate IPs aren't always the most important form of separation. Each client's sending domain/subdomain, authentication, list hygiene, engagement, complaint rate, and sending behavior matter enormously. A dedicated IP with poor sending practices can actually be worse than a well-managed shared pool.

If you tell me roughly how many clients you manage, monthly sends per client, and whether these are B2B or B2C, I can give you a much more specific architecture (including domains vs subdomains, dedicated IPs, and which platform I'd pick).

If your priority is multiple client accounts with genuinely separate sending reputations, I’d prioritize platforms that give each client a separate account/workspace, rather than putting every client into one shared database.

My shortlist

1. activecampaign.com — best overall for agencies

• Its Agency Program is specifically designed around resold separate client accounts, with centralized agency management. ActiveCampaign Help Center ActiveCampaign Help Center
• You can manage many client accounts under the agency relationship and get volume discounts as you scale. ActiveCampaign Help Center
• Strong automation, segmentation, CRM, and lifecycle marketing.
• Best if: you're a full-service agency managing 5–50+ clients and want serious automation.
• campaignmonitor.com — best for clean client separation

• Campaign Monitor explicitly supports separate client accounts, each with its own subscribers, campaigns, templates, analytics, permissions, and sending settings. Campaign Monitor
• That's a very good architectural fit if deliverability isolation is more important than having the deepest automation/CRM.
• Best if: you mainly manage newsletters, campaigns, and straightforward automations for clients.
• customer.io — best for sophisticated lifecycle/behavioral email

• Customer.io supports multiple workspaces under one account, with separate workspace environments for data and messaging. Customer.io Customer.io
• It has strong sending-domain controls and supports dedicated IPs as well as shared pools. Customer.io
• Best if: your agency does sophisticated product/lifecycle email rather than primarily newsletters.

One important distinction

Separate client accounts ≠ automatically separate IP reputation.

For what you're describing, I'd want each client to have:

Client A → separate account/workspace → Client A sending domain → appropriate IP/domain reputation

Client B → separate account/workspace → Client B sending domain → appropriate IP/domain reputation

rather than:

Agency account → Client A + B + C lists → shared sending infrastructure

Also, don't confuse a separate From address with reputation isolation. For example, Customer.io can support multiple From addresses, but its documentation shows that the underlying envelope-from/sending infrastructure is still something you need to configure deliberately. Customer.io

What I'd choose

My default recommendation: ActiveCampaign Agency Program. It is unusually well aligned with the agency model because the vendor actually structures the product around multiple resold client accounts rather than asking you to kludge multiple clients into one account. ActiveCampaign Help Center ActiveCampaign Help Center

If you tell me roughly how many clients you manage (e.g. 10, 50, 200), monthly email volume, and whether they're mostly e-commerce/B2B/local businesses, I can narrow this down to the best 2–3 options and compare account isolation, IP/domain reputation, white-labeling, pricing, and agency margins.

If your key requirement is one agency login with multiple client accounts and genuinely separate sending reputations, I’d shortlist these:

My first choice: Mailjet

mailjet.com is particularly interesting for your exact requirement. Its agency offering uses subaccounts, with each client's contacts, campaigns, domains and sending reputation isolated. It also lets clients have access to their own statistics without exposing other clients. Sinch Mailjet

That gives you a structure like:

AGENCY MASTER ACCOUNT
│
├── Client A
│   ├── clientA.com sending domain
│   ├── DKIM/SPF/DMARC
│   ├── contacts
│   └── reputation
│
├── Client B
│   ├── clientB.com sending domain
│   ├── DKIM/SPF/DMARC
│   ├── contacts
│   └── reputation
│
└── Client C
    ├── clientC.com sending domain
    ├── DKIM/SPF/DMARC
    ├── contacts
    └── reputation

That's much better than putting all clients into one ESP account and simply separating them with lists/tags.

If automation is the priority: ActiveCampaign

activecampaign.com would be my second choice. It's stronger if your agency is selling complex automation, CRM, lead nurturing and lifecycle marketing rather than primarily newsletter/campaign execution. Current agency-focused comparisons also highlight its multi-account/partner capabilities. Best Email Reviews Best Email Reviews

If you're managing a lot of smaller clients: Brevo

brevo.com is worth looking at if you expect dozens of relatively small accounts and care heavily about controlling cost. Its agency/sub-organization model is designed for this sort of portfolio management. Sasanova

One important distinction

If by "separate sending reputations" you mean cold outbound email/lead generation, rather than permission-based marketing/newsletters, I'd choose a very different stack.

For cold email, I would not simply put 20 clients into one conventional ESP. Each client should have their own sending domains, authentication, mailboxes/infrastructure and reputation. Quarvio Quarvio

For opt-in marketing email, my ranking would be:

1. Mailjet → best fit for your stated requirement
2. ActiveCampaign → best if automation/CRM is central
3. Brevo → best for many smaller clients / cost efficiency
4. Elastic Email → worth considering for high-volume operations

If you tell me roughly how many clients you manage (e.g. 10, 50, 200), monthly sends, and whether this is opt-in marketing or cold email, I can narrow it down to the best 2–3 architectures and compare the likely cost.

Yes. For an agency, I’d prioritize account isolation and sender-reputation isolation over the usual “best email builder” features.

My shortlist

My pick: Campaign Monitor

If your core requirement is:

“I want one agency login, multiple completely separate client accounts, and each client should have its own lists, domain, reputation and reporting.”

Campaign Monitor is particularly well suited. Its agency/client architecture is explicitly designed around isolated client accounts, including separate sending domains. Hack'celeration

That's preferable to putting 20 clients into one giant account and merely separating them with lists/tags.

If you're doing sophisticated automation: ActiveCampaign

I'd choose ActiveCampaign if you're selling things like:

• Lead nurturing
• Complex behavioral automation
• CRM integrations
• Sales pipelines
• Lifecycle campaigns
• Automated onboarding sequences

It has strong agency/partner functionality and much deeper automation capabilities. BestSends Sasanova

The important caveat is that I'd still create a genuinely separate account/environment per client, rather than relying on lists inside one account.

If you want an agency operating system: HighLevel

HighLevel is worth serious consideration if you're running an agency where email is only one part of the service.

Its sub-account architecture is very agency-friendly, and its current email infrastructure supports dedicated sending domains at the sub-account level. You can also assign different sending domains to different types of email. HighLevel Support Portal HighLevel Support Portal

So you could have:

AGENCY
│
├── Client A
│   └── send.clientA.com
│
├── Client B
│   └── send.clientB.com
│
├── Client C
│   └── send.clientC.com
│
└── Client D
    └── send.clientD.com

That's a much better architecture than:

AGENCY
└── one shared sending domain
    ├── Client A
    ├── Client B
    ├── Client C
    └── Client D

because a problematic sender shouldn't unnecessarily affect the others.

One important distinction: domain vs. IP reputation

I'd actually recommend separating clients by sending domain/subdomain first, rather than obsessing over dedicated IPs.

For example:

send.clientA.com

send.clientB.com

send.clientC.com

A dedicated IP can make sense at sufficiently high volume, but it isn't automatically better for smaller senders because the IP itself needs enough volume and a good sending history. HighLevel, for example, recommends dedicated IPs particularly for very high-volume sending. HighLevel Support Portal

Klaviyo also explicitly supports branded sending domains and separate domains for marketing, transactional, and service traffic, allowing those streams to develop separate reputations. Klaviyo Help Center

What I'd choose by agency type

• 10–50 clients, mostly newsletters/campaigns: Campaign Monitor
• 10–30 clients with sophisticated automation: ActiveCampaign
• Full-service marketing agency / CRM + funnels + email + SMS: HighLevel
• E-commerce clients: Klaviyo, with a separate account per client
• Lots of small clients and cost is critical: Brevo

One thing I would not do is put all clients under your agency's primary sending domain. Even if the platform technically lets you segment them, you're creating unnecessary reputational coupling.

If you tell me roughly how many clients you manage, monthly email volume per client, and whether these are B2B, e-commerce, or local businesses, I can narrow this down to the best 2–3 architectures and estimate what the agency setup should look like.

So you could have:

AGENCY
│
├── Client A
│   └── send.clientA.com
│
├── Client B
│   └── send.clientB.com
│
├── Client C
│   └── send.clientC.com
│
└── Client D
    └── send.clientD.com

That's a much better architecture than:

AGENCY
└── one shared sending domain
    ├── Client A
    ├── Client B
    ├── Client C
    └── Client D

because a problematic sender shouldn't unnecessarily affect the others.

One important distinction: domain vs. IP reputation

I'd actually recommend separating clients by sending domain/subdomain first, rather than obsessing over dedicated IPs.

For example:

send.clientA.com

send.clientB.com

send.clientC.com

A dedicated IP can make sense at sufficiently high volume, but it isn't automatically better for smaller senders because the IP itself needs enough volume and a good sending history. HighLevel, for example, recommends dedicated IPs particularly for very high-volume sending.

Klaviyo also explicitly supports branded sending domains and separate domains for marketing, transactional, and service traffic, allowing those streams to develop separate reputations.

What I'd choose by agency type

Yes. If your key requirement is one agency managing many clients while keeping each client’s sending reputation isolated, I’d narrow the field considerably.

My top picks

1. Campaign Monitor — probably the cleanest fit

Campaign Monitor is particularly well suited to the agency model. Its agency setup lets you manage separate client accounts, with each client retaining separate subscribers, campaigns, analytics and sending settings. Campaign Monitor Hack'celeration

That's important because I would not put all your clients' lists into one giant account and merely segment them. You want an architectural boundary between clients.

I'd choose Campaign Monitor if: your agency primarily provides newsletters, lifecycle campaigns, design, segmentation and reporting.

2. Mailjet — worth a serious look

Mailjet explicitly supports agency subaccounts, with client data, lists and sending reputation isolated while allowing the agency to manage everything centrally. Sinch Mailjet

That's very close to what you're describing.

I'd choose Mailjet if: centralized agency management and clean client separation are more important than having the deepest marketing-automation ecosystem.

3. HighLevel — excellent if you're an automation/CRM agency

HighLevel is compelling because each client can live in its own sub-account, and HighLevel supports dedicated sending domains.

Their current guidance is particularly relevant to your concern: they recommend not sharing a dedicated sending domain across sub-accounts, specifically to maintain isolated deliverability and avoid cross-account reputation problems. HighLevel Support Portal

So an architecture could look like:

YOUR AGENCY
│
├── Client A
│   ├── clientA.com
│   └── email.clientA.com
│
├── Client B
│   ├── clientB.com
│   └── email.clientB.com
│
├── Client C
│   ├── clientC.com
│   └── email.clientC.com
│
└── Client D
    ├── clientD.com
    └── email.clientD.com

That's much closer to how I'd structure an agency operation.

One important distinction: domain reputation vs. IP reputation

This is where I'd be careful.

Separate accounts alone don't necessarily mean completely independent deliverability. You ideally want each client to have:

• Their own sending domain/subdomain
• SPF
• DKIM
• DMARC
• Separate suppression/list data
• Appropriate sending volume
• Ideally an isolated IP when the client's volume warrants it

HighLevel, for example, distinguishes between a dedicated sending domain and a dedicated IP; even with a dedicated domain, you can still be sharing an IP pool. Their documentation says dedicated IPs become particularly relevant at high volume. HighLevel Support Portal

So if by "separate sending reputations" you mean "Client A getting blacklisted should have essentially zero impact on Client B," I'd make dedicated domains/subaccounts a minimum and investigate dedicated IP arrangements for larger clients.

What I would pick

For an agency, I'd rank them:

Campaign Monitor → Mailjet → HighLevel → ActiveCampaign → Brevo

But there's an important caveat:

• Newsletter/email marketing agency: Campaign Monitor
• Email + CRM + funnels + automation agency: HighLevel
• Advanced lifecycle/behavioral automation: ActiveCampaign
• Lots of smaller clients / margin-sensitive: Mailjet or Brevo
• E-commerce clients: I'd actually consider Klaviyo separately for each client, rather than forcing everyone onto one agency-wide ESP.

If you tell me roughly how many clients you manage (e.g. 10, 50, 200), average emails/month per client, and whether this is newsletters vs. marketing automation vs. cold email, I can give you a much more definitive recommendation—including which platforms actually let you isolate reputation and which only look like they do.

That's important because I would not put all your clients' lists into one giant account and merely segment them. You want an architectural boundary between clients.

I'd choose Campaign Monitor if: your agency primarily provides newsletters, lifecycle campaigns, design, segmentation and reporting.

2. Mailjet — worth a serious look

Mailjet explicitly supports agency subaccounts, with client data, lists and sending reputation isolated while allowing the agency to manage everything centrally. Sinch Mailjet

That's very close to what you're describing.

I'd choose Mailjet if: centralized agency management and clean client separation are more important than having the deepest marketing-automation ecosystem.

3. HighLevel — excellent if you're an automation/CRM agency

HighLevel is compelling because each client can live in its own sub-account, and HighLevel supports dedicated sending domains.

Their current guidance is particularly relevant to your concern: they recommend not sharing a dedicated sending domain across sub-accounts, specifically to maintain isolated deliverability and avoid cross-account reputation problems. help.gohighlevel.com

So an architecture could look like:

YOUR AGENCY
│
├── Client A
│   ├── clientA.com
│   └── email.clientA.com
│
├── Client B
│   ├── clientB.com
│   └── email.clientB.com
│
├── Client C
│   ├── clientC.com
│   └── email.clientC.com
│
└── Client D
    ├── clientD.com
    └── email.clientD.com

That's much closer to how I'd structure an agency operation.

One important distinction: domain reputation vs. IP reputation

This is where I'd be careful.

For an agency, the “best” email marketing platform is usually not the one with the best editor or automation features — it’s the one that gives you client isolation, separate sender reputations, clean permissions, and scalable operations. A good agency setup should avoid putting multiple clients into one shared sending environment because one client’s poor list quality or complaint rate can hurt others. InboxAlly Mailpro

Here are the strongest options by agency use case:

Hack'celeration Sasanova Sinch Mailjet## My shortlist for an agency

1. ActiveCampaign — best overall for most agencies

Use this if you manage:

• SaaS clients
• B2B lead generation
• coaches/consultants
• service businesses needing nurture sequences

Why:

• Separate client accounts are possible through agency/reseller structures
• Excellent automation builder
• Strong CRM and lead scoring
• Good balance between agency control and client ownership

A typical setup:

• Client A → their own ActiveCampaign account → their own domain authentication
• Client B → separate account → separate reputation
• Agency team manages both

BestSends PG Systems Lab### 2. Campaign Monitor — best for pure email management

Good fit if your agency mostly does:

• newsletters
• promotions
• editorial emails
• nonprofit/community communications

It is built more around managing multiple brands and client campaigns than complex sales automation. Hack'celeration

3. Brevo — best for lots of smaller clients

If you have 20–100 smaller accounts (restaurants, local businesses, clinics, etc.), Brevo’s pricing model and account structure can make more sense than contact-based pricing platforms. Sasanova

4. SendGrid / Mailjet — best when deliverability isolation is the priority

If your agency sends:

• transactional email
• high-volume campaigns
• application-triggered emails

then infrastructure-first platforms can be attractive because you can separate clients using subaccounts, domains, and sending configurations. Sinch Mailjet The Open Rate Club

The setup I would recommend for protecting reputation

Regardless of platform:

• One sending domain per client

• Example: news.clientdomain.com
• Not: agencydomain.com/client1
• Separate SPF, DKIM, and DMARC authentication per client
• Separate client lists

• Never mix subscribers across clients
• Separate suppression/unsubscribe handling
• Dedicated IP only when volume justifies it

• A low-volume client usually benefits more from a healthy shared pool than an unused dedicated IP
• Client owns the sending domain

• Makes offboarding cleaner

My practical recommendation

For a typical marketing agency:

• 5–30 clients: ActiveCampaign + separate client accounts
• 30–100 smaller clients: Brevo or Mailjet-style subaccount model
• E-commerce agency: Klaviyo
• Enterprise/B2B retainers: HubSpot or ActiveCampaign
• Newsletter agency: Campaign Monitor

The biggest mistake is putting all clients into one master ESP account with different lists. It looks convenient early, but it creates reputation, billing, permissions, and offboarding problems later. Mailpro

If you tell me your agency model (lead gen, e-commerce, newsletters, cold email, SaaS, local businesses) and approximate number of clients/sends per month, I can narrow this to a specific stack.

If your main requirement is an agency managing multiple clients while keeping each client’s sending reputation isolated, I’d narrow the field considerably.

My ranking

1. Mailgun — best for true reputation isolation

• Has actual subaccounts, each with separate sending domains, API keys, users, data and reporting. Mailgun Documentation Mailgun Documentation
• You can assign dedicated IP pools per client, which is particularly useful when one client has poor engagement or questionable sending practices. Mailgun Help Center Mailgun Documentation
• Mailgun explicitly describes IP pools as an agency/reseller use case for keeping Client 1's sending separate from Client 2's. Mailgun Help Center
• Best if your agency is comfortable with a more technical/API-oriented platform.

Verdict: ⭐⭐⭐⭐⭐ for deliverability architecture

2. ActiveCampaign — best for agency marketing automation

• Its Partner Portal is specifically designed for agencies/resellers managing multiple client accounts. ActiveCampaign Help Center
• Each client can have their own account and authenticated sending domain; ActiveCampaign supports DKIM, SPF and DMARC setup. ActiveCampaign Help Center
• Much better choice than Mailgun if your clients need sophisticated CRM + automations + campaigns rather than primarily an email-delivery infrastructure.
• The important distinction: I'd use separate client accounts, rather than trying to make one ActiveCampaign account behave like multiple isolated senders.

Verdict: ⭐⭐⭐⭐½ for agency usability

3. Mailjet — worth considering for a simpler agency setup

• Mailjet has an agency-oriented architecture with subaccounts, designed to isolate client data, lists and sending reputation while allowing centralized management. Sinch Mailjet
• This is attractive if you want something less infrastructure-heavy than Mailgun.

Verdict: ⭐⭐⭐⭐

The architecture I'd recommend

For an agency, I wouldn't simply put all clients into one shared email-marketing account.

I'd structure it approximately like:

AGENCY
│
├── Client A
│   ├── Separate account/subaccount
│   ├── clientA.com / mail.clientA.com
│   ├── DKIM/SPF/DMARC
│   └── Client A sending reputation
│
├── Client B
│   ├── Separate account/subaccount
│   ├── clientB.com / mail.clientB.com
│   ├── DKIM/SPF/DMARC
│   └── Client B sending reputation
│
└── Client C
    ├── Separate account/subaccount
    ├── clientC.com / mail.clientC.com
    ├── DKIM/SPF/DMARC
    └── Client C sending reputation

That's important because reputation isn't just an IP issue—domain reputation matters too. Mailgun, for example, recommends separating marketing and transactional mail onto different domains/subdomains, and notes that separate domains can help isolate reputation. Mailgun Documentation Mailgun

One important caveat

Separate sending domains are more important than simply buying separate IPs. A dedicated IP doesn't magically give a client good deliverability. You still need authentication, appropriate volume ramp-up, low complaint rates, good list hygiene, and engagement.

If you're talking about an agency with 10–100+ client accounts, I'd lean strongly toward Mailgun's subaccount/IP-pool architecture for the sending layer, potentially paired with a separate marketing automation platform.

If you tell me roughly how many clients you manage, monthly email volume per client, and whether you need CRM/automation, I can give you a much more specific recommendation between Mailgun vs ActiveCampaign vs Mailjet vs HubSpot vs Klaviyo.

If your priority is one agency managing many clients while keeping each client’s sending reputation isolated, I’d shortlist platforms differently than I would for a single-brand marketer.

My picks

Mailjet is particularly interesting for your exact requirement: its agency offering explicitly supports subaccounts with separate client data, lists and sending reputations, while letting the agency manage them from one platform. Sinch Mailjet

The important distinction: account isolation ≠ reputation isolation

This is the part I'd pay the most attention to.

You don't merely want:

Agency → Client A workspace / Client B workspace / Client C workspace

You want:

Client A → its own authenticated sending domain + reputation
Client B → its own authenticated sending domain + reputation
Client C → its own authenticated sending domain + reputation

Ideally, each client can also have a dedicated IP when their volume and deliverability requirements justify it.

Otherwise, a client with poor list hygiene or spam complaints can potentially create problems for other clients sharing infrastructure. Agency-focused comparisons also flag dedicated IPs and sub-account isolation as key criteria. Sasanova

What I'd choose by agency type

If you're primarily doing newsletters + lifecycle marketing for many different businesses:
→ Mailjet

If you're building sophisticated automations, CRM pipelines, lead nurturing, etc.:
→ ActiveCampaign

If you have 20–100+ relatively small clients and care heavily about cost:
→ Brevo

If most clients are Shopify/ecommerce brands:
→ Klaviyo, with a separate Klaviyo account per client.

If you actually mean cold outbound rather than opted-in marketing email:
That's a different category entirely. I'd use dedicated outbound infrastructure rather than a conventional ESP. Some newer agency platforms explicitly provision separate infrastructure/IPs per client. SendKit SendChief

One architecture I'd recommend

For an agency, I'd structure it like:

AGENCY
│
├── Client A
│   ├── clienta.com sending domain
│   ├── Client A contacts/suppressions
│   ├── Client A automations
│   └── Client A reporting
│
├── Client B
│   ├── clientb.com sending domain
│   ├── Client B contacts/suppressions
│   ├── Client B automations
│   └── Client B reporting
│
└── Client C
    ├── clientc.com sending domain
    ├── Client C contacts/suppressions
    ├── Client C automations
    └── Client C reporting

I would not put all clients' contacts into one giant account/list, even if the platform technically allows it.

If you tell me roughly how many clients you manage (e.g. 10, 50, 200), monthly sends per client, and whether this is opt-in marketing or cold email, I can narrow this down to the best 2–3 platforms and explain the infrastructure I'd use for keeping reputations genuinely separate.

Yes. If your priority is managing multiple clients while keeping each client’s sending reputation genuinely separate, I’d narrow the field to ActiveCampaign, Brevo, Mailjet, and Mailchimp—but they solve the problem differently.

My shortlist

• Brevo's strongest multi-client/sub-organization capabilities are on Enterprise. Its documentation specifically describes separate sub-organizations, contact lists, API keys, permissions, and the ability to use separate dedicated IPs. Brevo Help Brevo Help

🥇 My pick: ActiveCampaign

For a marketing agency running campaigns + automations for multiple clients, I'd probably start with ActiveCampaign.

Its agency program is designed around separate client accounts, rather than putting 15 clients into one giant account and trying to separate them with tags/lists. That is much cleaner operationally and for offboarding clients. PG Systems Lab

The big advantage is automation: if you're doing lead nurturing, abandoned-cart-style workflows, behavioral triggers, lead scoring, CRM integration, etc., ActiveCampaign is considerably stronger than the simpler platforms.

The architecture I'd use:

Agency → Client A account → Client A sending domain
Agency → Client B account → Client B sending domain
Agency → Client C account → Client C sending domain

That gives you much better separation than:

One agency account → 10 clients → 10 lists → 10 domains

The latter can work technically, but it's not how I'd structure an agency where deliverability and client isolation are important.

🥇 If reputation isolation is the #1 concern: Mailjet or Brevo

This is where things get interesting.

mailjet.com explicitly positions its subaccounts around keeping each client's data, lists, and sending reputation isolated, while letting the agency manage them from one platform. Sinch Mailjet

Brevo's Enterprise sub-organization model similarly gives each client a separate environment, including separate contacts, API keys, permissions, and the option for separate dedicated IPs. Brevo Help

If you're managing, say, 20–50 clients, I'd seriously investigate these before defaulting to ActiveCampaign.

One important distinction: domain reputation vs IP reputation

If by "separate sending reputations" you mean you don't want Client A's bad list or spam complaints to hurt Client B, I'd prioritize:

1. Separate client accounts/subaccounts
2. Separate authenticated sending domains/subdomains
3. Separate tracking infrastructure where appropriate
4. Separate IPs if your volume justifies dedicated IPs
5. Good suppression/bounce handling

A dedicated IP alone doesn't magically solve reputation isolation. Domain reputation is also important.

For example, Brevo recommends that a sending domain not be simultaneously used on another email platform, and lets multiple domains be configured independently. Brevo Help

What I would not do

I wouldn't put all your clients into one normal Mailchimp/ActiveCampaign/Brevo account and simply separate them with lists/tags.

That's convenient initially, but it creates headaches around:

• client access
• billing
• data separation
• reporting
• client offboarding
• accidental sends
• domain configuration
• reputation management

Mailchimp does have an agency-oriented connected-account system, so an agency can switch between clients' separate accounts without sharing credentials. Mailchimp

My recommendation by agency type

• 5–15 clients, sophisticated automations: ActiveCampaign
• 15–50+ clients, strong need for account isolation: Brevo Enterprise or Mailjet
• E-commerce clients: I'd consider Klaviyo per client, rather than forcing everyone into one platform. Klaviyo supports branded sending domains and independent DNS configuration. Klaviyo Help Center
• Clients already familiar with Mailchimp: Mailchimp can be perfectly reasonable; use separate client accounts.
• Agency wants to resell/manage everything under its own umbrella: ActiveCampaign's agency program is particularly worth investigating.

If you tell me roughly how many clients you manage, monthly email volume per client, and whether they're mostly e-commerce/B2B/local businesses, I can recommend the exact architecture (including domains, subaccounts, dedicated IPs, billing, and client access) I'd use.

The big advantage is automation: if you're doing lead nurturing, abandoned-cart-style workflows, behavioral triggers, lead scoring, CRM integration, etc., ActiveCampaign is considerably stronger than the simpler platforms.

The architecture I'd use:

Agency → Client A account → Client A sending domain
Agency → Client B account → Client B sending domain
Agency → Client C account → Client C sending domain

That gives you much better separation than:

One agency account → 10 clients → 10 lists → 10 domains

The latter can work technically, but it's not how I'd structure an agency where deliverability and client isolation are important.

🥇 If reputation isolation is the #1 concern: Mailjet or Brevo

This is where things get interesting.

mailjet.com explicitly positions its subaccounts around keeping each client's data, lists, and sending reputation isolated, while letting the agency manage them from one platform. mailjet.com

Brevo's Enterprise sub-organization model similarly gives each client a separate environment, including separate contacts, API keys, permissions, and the option for separate dedicated IPs.

If you're managing, say, 20–50 clients, I'd seriously investigate these before defaulting to ActiveCampaign.

One important distinction: domain reputation vs IP reputation

Yes. For an agency, I’d prioritize account isolation and deliverability architecture over the usual “best email editor” criteria.

My shortlist

Campaign Monitor Sinch Mailjet Sasanova AhaSend### My pick: Mailjet or AhaSend

Mailjet is particularly interesting for your use case because its agency setup uses subaccounts, with each client's data, lists, and sending reputation isolated, while the agency retains centralized management. Sinch Mailjet

AhaSend is even more compelling if your main concern is true sending separation. Its newly introduced subaccounts are complete accounts with their own domains, API keys, suppressions, and sending reputation, while remaining under one parent billing relationship. AhaSend

That's very close to the architecture I'd want for an agency.

One important distinction

When you say “separate sending reputations,” I would not settle for merely having separate lists/workspaces.

Ideally each client should have:

• Their own sending domain/subdomain
• Their own DKIM/SPF/DMARC configuration
• Their own suppression/bounce/complaint history
• Their own reputation
• Ideally their own dedicated IP or appropriately isolated IP pool if volume warrants it
• Separate API credentials
• Separate client-level permissions
• The ability to remove/transfer the client cleanly later

This matters because a client with poor list hygiene or high complaint rates shouldn't create unnecessary deliverability risk for your other clients. Mailpro Mailsoftly

If you're primarily doing marketing automation

I'd lean ActiveCampaign. Its agency/partner infrastructure is strong and its automation capabilities are considerably deeper than Mailjet/AhaSend. Sasanova

If you're primarily an email-sending agency

I'd lean Mailjet or AhaSend.

The distinction is basically:

ActiveCampaign = marketing automation platform with agency capabilities

Mailjet = email marketing/sending platform with agency capabilities

AhaSend = sending infrastructure with multi-tenant isolation

If you tell me roughly how many clients you manage, monthly sends per client, and whether you're doing newsletters vs. cold/lead-gen vs. lifecycle automation, I can narrow this down to the best 2–3 architectures, including whether you should use shared or dedicated IPs.

If your main requirement is an agency managing multiple client accounts where each client needs a genuinely separate sending reputation, I’d narrow it to Campaign Monitor, Mailjet, ActiveCampaign, and Brevo—but for somewhat different reasons.

My ranking for an agency

🥇 Campaign Monitor

This would be my first platform to investigate for your use case.

Campaign Monitor specifically supports agencies managing multiple client accounts, with each client account kept separate while the agency manages them centrally. Their agency offering is explicitly designed around multiple brands, audiences, campaigns, automations and reporting. Campaign Monitor Hack'celeration

The important part for you is that Client A's account isn't just another list inside the same account as Client B. You can maintain separate client environments and sending configurations.

campaignmonitor.com

🥈 Mailjet

Mailjet is particularly interesting if reputation isolation is the #1 requirement.

Their agency offering uses subaccounts, with client data, lists and sending reputation isolated between accounts, while allowing the agency to manage everything from one platform. Sinch Mailjet

That architecture is very close to what you're describing.

mailjet.com

🥉 ActiveCampaign

I'd choose ActiveCampaign if your agency is selling sophisticated automation rather than primarily newsletters.

For example:

• Lead scoring
• Complex nurture sequences
• CRM integration
• Behavioral triggers
• Sales automation
• Multi-step conditional workflows

The tradeoff is that the agency/account architecture and pricing can become more complicated as you add clients. Independent 2026 comparisons also identify ActiveCampaign as particularly strong for automation-focused agencies. Hack'celeration Sasanova

activecampaign.com

What I'd do for client reputation

This is actually more important than the ESP itself.

I would structure it roughly like:

YOUR AGENCY
│
├── Client A
│   ├── Separate ESP account/subaccount
│   ├── clientA.com
│   ├── DKIM/SPF/DMARC
│   └── Client A sending reputation
│
├── Client B
│   ├── Separate ESP account/subaccount
│   ├── clientB.com
│   ├── DKIM/SPF/DMARC
│   └── Client B sending reputation
│
└── Client C
    ├── Separate ESP account/subaccount
    ├── clientC.com
    ├── DKIM/SPF/DMARC
    └── Client C sending reputation

Don't just create three lists inside one agency account and assume that gives you reputation isolation. The important separation is the sending domain/account infrastructure.

For example, Klaviyo explicitly notes that a branded sending domain's reputation can be affected by multiple Klaviyo accounts using that same domain. It also supports separate branded domains for marketing vs. transactional traffic. Klaviyo Help Center Klaviyo Help Center

One important distinction

If by "separate sending reputations" you mean cold email/outbound prospecting, rather than permission-based newsletters/marketing email, my answer changes substantially.

For cold email, I'd look at a different category of platforms—tools built around multiple mailboxes/domains, client workspaces, warm-up, and reputation isolation. Traditional ESPs such as Campaign Monitor/Klaviyo aren't necessarily the right architecture.

If you're doing newsletter + lifecycle/marketing email for clients, my shortlist would be:

Campaign Monitor → Mailjet → ActiveCampaign → Brevo

If you tell me roughly how many clients you manage (5, 20, 50+), monthly sends per client, and whether this is marketing email or cold outreach, I can narrow it down to the best 1–2 platforms and explain the account/reputation architecture I'd use.

Yes. For an agency, I’d prioritize account isolation over the usual “best email marketing platform” criteria. You want each client to have its own contacts, sending domain, authentication, reputation, billing, and ideally IP reputation.

My shortlist

1. Klaviyo — my top pick for ecommerce agencies

klaviyo.com

Klaviyo is particularly strong if every client gets a completely separate Klaviyo account. Each account is a self-contained environment with its own profiles, lists, flows, integrations, billing, and sending domains. Klaviyo also has portfolios/organizations for managing multiple accounts centrally. Klaviyo Help Center Klaviyo Help Center

Most importantly for your concern, you can use branded sending domains, so each client's mail can be authenticated against their own domain. Klaviyo explicitly says branded domains give you greater control over sender reputation. Klaviyo Help Center

One caveat: don't put all clients into one Klaviyo account if reputation isolation is important. Separate accounts are the cleaner architecture.

2. ActiveCampaign — probably the best general-purpose agency choice

activecampaign.com

I'd strongly consider this if your agency does more than newsletters—e.g. lead nurturing, sales automation, CRM, onboarding, behavioral campaigns, etc.

ActiveCampaign supports agency/reseller management through its Partner Portal, and its sending infrastructure lets accounts use their own sending domains/mailserver domains. ActiveCampaign Help Center ActiveCampaign Help Center

It also explicitly recommends using subdomains to help protect and build sending reputation across different mail streams. ActiveCampaign Help Center

A typical setup could be:

• Client A → send.clientA.com
• Client B → send.clientB.com
• Client C → send.clientC.com

rather than having every client effectively send through the agency's domain.

3. Mailchimp — surprisingly good for traditional agency management

mailchimp.com

Mailchimp has a specific agency-oriented mechanism for connecting and switching between client accounts without sharing credentials. Mailchimp

And, importantly, Mailchimp itself has historically recommended a separate account for each client, specifically because mixing clients creates data and account-risk problems. Mailchimp Downloads

Mailchimp is therefore a solid choice if your clients primarily need conventional campaigns rather than extremely sophisticated automation.

The architecture I'd use

If your primary requirement is "Client A must not damage Client B's sending reputation", I'd structure it like this:

YOUR AGENCY
│
├── Client A
│   ├── Separate platform account
│   ├── clientA.com
│   ├── send.clientA.com
│   ├── DKIM
│   ├── SPF
│   └── DMARC
│
├── Client B
│   ├── Separate platform account
│   ├── clientB.com
│   ├── send.clientB.com
│   ├── DKIM
│   ├── SPF
│   └── DMARC
│
└── Client C
    ├── Separate platform account
    ├── clientC.com
    ├── send.clientC.com
    ├── DKIM
    ├── SPF
    └── DMARC

I would not create one giant agency account with a separate list for each client. The account/domain/reputation boundaries are much cleaner when each client has their own account.

One subtle point: separate sending domains don't necessarily mean completely separate IP reputation. For example, Klaviyo notes that most accounts initially use shared IP infrastructure; dedicated IPs are a separate consideration for sufficiently high-volume senders. Klaviyo Help Center

What I'd choose

• Ecommerce agency: Klaviyo
• B2B / lead-gen / automation agency: ActiveCampaign
• General-purpose agency serving many SMBs: Mailchimp
• Lots of clients + price sensitivity: Brevo
• High-volume clients where IP isolation matters: I'd evaluate the platform's dedicated-IP/subaccount architecture separately rather than choosing purely on marketing features.

If you tell me roughly how many client accounts you manage (e.g. 10, 50, 200), average monthly email volume, and whether they're B2B or ecommerce, I can narrow this down to the best 2–3 and compare account isolation, domains, dedicated IPs, white-labeling, API access, billing, and agency pricing.

Mailchimp has a specific agency-oriented mechanism for connecting and switching between client accounts without sharing credentials. mailchimp.com

And, importantly, Mailchimp itself has historically recommended a separate account for each client, specifically because mixing clients creates data and account-risk problems. downloads.mailchimp.com

Mailchimp is therefore a solid choice if your clients primarily need conventional campaigns rather than extremely sophisticated automation.

The architecture I'd use

If your primary requirement is "Client A must not damage Client B's sending reputation", I'd structure it like this:

YOUR AGENCY
│
├── Client A
│   ├── Separate platform account
│   ├── clientA.com
│   ├── send.clientA.com
│   ├── DKIM
│   ├── SPF
│   └── DMARC
│
├── Client B
│   ├── Separate platform account
│   ├── clientB.com
│   ├── send.clientB.com
│   ├── DKIM
│   ├── SPF
│   └── DMARC
│
└── Client C
    ├── Separate platform account
    ├── clientC.com
    ├── send.clientC.com
    ├── DKIM
    ├── SPF
    └── DMARC

ActiveCampaign supports agency/reseller management through its Partner Portal, and its sending infrastructure lets accounts use their own sending domains/mailserver domains.

If by “separate sending reputations” you mean each client should have its own sending domain/subdomain, lists, suppression data, and ideally IP reputation, I’d narrow it to a few platforms.

My shortlist

My first choice: Campaign Monitor

For a traditional marketing agency, Campaign Monitor is particularly attractive because its agency-oriented setup is designed around separate client accounts rather than putting every client into one giant database. Recent agency-focused comparisons specifically highlight isolated client accounts, separate sending domains, and centralized agency management. Hack'celeration

That architecture is important: if Client A gets a bunch of spam complaints, you don't want Client B effectively sharing the same sending identity.

If automation is the priority: ActiveCampaign

I'd choose ActiveCampaign if you're selling things like:

• lead nurturing
• behavioral automations
• CRM integration
• lead scoring
• complex customer journeys
• automated follow-up sequences

Its agency program is designed around managing multiple client accounts, while its deliverability documentation recommends using subdomains to segment different email streams and reputations. ActiveCampaign Help Center

The downside is that it's more expensive/complex if all you're doing is newsletters.

If you have e-commerce clients: Klaviyo

For Shopify/e-commerce clients, I'd strongly consider Klaviyo rather than forcing everything into one generic ESP.

Klaviyo supports branded sending domains and separate domains for marketing, transactional, and service traffic. Klaviyo Help Center Klaviyo Help Center

One important caveat for an agency: each Klaviyo account can have its own sending setup, but sharing a branded sending domain across multiple Klaviyo accounts means those accounts can still affect that domain's reputation. Klaviyo Help Center So I'd give each client their own sending subdomain/domain rather than trying to share one agency domain.

An underrated option: Mailjet

If true client isolation is your #1 requirement, Mailjet is worth investigating. Its agency offering explicitly uses subaccounts, with client data, lists, and sending reputation isolated while the agency manages them from one platform. Sinch Mailjet

That is very close to what you're describing.

One important distinction

If you're talking about cold outbound email, rather than opted-in marketing/newsletters, my recommendation changes substantially.

For cold email, I would not put 10 clients into one conventional marketing ESP and assume separate “accounts” automatically mean separate reputations. I'd build something more like:

Client → dedicated sending domain(s) → dedicated mailboxes/infrastructure → isolated campaign/suppression data

That is a different infrastructure problem from newsletter/marketing email. Quarvio

So, my ranking for a typical agency:

1. Campaign Monitor — best overall for multi-client campaign management
2. ActiveCampaign — best if automation/CRM is central to your service
3. Mailjet — best if strict subaccount/reputation isolation is the priority
4. Brevo — best value when you have lots of smaller clients
5. Klaviyo — best if most clients are e-commerce

If you tell me roughly how many clients you manage (e.g. 10, 50, 200), monthly send volume, and whether this is opt-in marketing or cold email, I can narrow it down to the best architecture and tell you how I'd structure the client accounts/domains/IPs.

For Shopify/e-commerce clients, I'd strongly consider Klaviyo rather than forcing everything into one generic ESP.

Klaviyo supports branded sending domains and separate domains for marketing, transactional, and service traffic. Klaviyo Help Center Klaviyo Help Center

One important caveat for an agency: each Klaviyo account can have its own sending setup, but sharing a branded sending domain across multiple Klaviyo accounts means those accounts can still affect that domain's reputation. Klaviyo Help Center So I'd give each client their own sending subdomain/domain rather than trying to share one agency domain.

An underrated option: Mailjet

If true client isolation is your #1 requirement, Mailjet is worth investigating. Its agency offering explicitly uses subaccounts, with client data, lists, and sending reputation isolated while the agency manages them from one platform. mailjet.com

That is very close to what you're describing.

One important distinction

If you're talking about cold outbound email, rather than opted-in marketing/newsletters, my recommendation changes substantially.

For cold email, I would not put 10 clients into one conventional marketing ESP and assume separate “accounts” automatically mean separate reputations. I'd build something more like:

Client → dedicated sending domain(s) → dedicated mailboxes/infrastructure → isolated campaign/suppression data

Yes. For an agency, I’d prioritize account/client isolation and sending-domain reputation isolation over raw email features.

My shortlist

🥇 My default choice: ActiveCampaign

For a typical marketing agency managing, say, 10–50 clients, I'd seriously consider ActiveCampaign.

The important architectural point is: don't put all clients into one sending account just because the platform allows it. Give each client their own account and their own authenticated sending domain/subdomain. ActiveCampaign specifically recommends using subdomains to segment email traffic and manage reputation. ActiveCampaign Help Center

That gives you:

• Client A's reputation isn't directly mixed with Client B's.
• Each client can have their own SPF/DKIM/DMARC configuration.
• You can hand an account back to a client cleanly if they leave.
• Client-specific automations, lists, integrations and analytics stay isolated.
• ActiveCampaign has an agency/partner model for managing multiple client accounts. Mailsoftly

The downside: costs can become significant as your client/contact count grows.

activecampaign.com

🛒 If your clients are ecommerce: Klaviyo

I'd choose Klaviyo instead if most of your clients are Shopify/ecommerce brands.

Klaviyo has particularly strong support for branded sending domains, and its infrastructure explicitly treats the sending domain as an important component of sender reputation. Klaviyo Help Center

It also now supports separate branded sending domains for marketing, transactional and service email, which is useful when you want to isolate different types of traffic. Klaviyo

One nuance: a Klaviyo account can only have one active branded sending domain per send type, so for true client isolation I'd use separate Klaviyo accounts rather than trying to put many clients into one account. Klaviyo Help Center

klaviyo.com

💰 Brevo is interesting for a high-volume agency

Brevo is worth looking at if you're managing many clients and send volume matters more than sophisticated automation.

Its sub-organization model is specifically useful for separating clients while retaining centralized agency administration/billing. Mailsoftly

brevo.com

One thing I'd strongly recommend

If by "separate sending reputations" you mean:

Client A has a bad campaign → I don't want Client B's deliverability affected

then I'd structure your agency like this:

AGENCY
│
├── Client A
│   ├── Separate platform account
│   ├── clientA.com / email.clientA.com
│   ├── SPF + DKIM + DMARC
│   └── Client A sending reputation
│
├── Client B
│   ├── Separate platform account
│   ├── clientB.com / email.clientB.com
│   ├── SPF + DKIM + DMARC
│   └── Client B sending reputation
│
└── Client C
    ├── Separate platform account
    ├── clientC.com / email.clientC.com
    ├── SPF + DKIM + DMARC
    └── Client C sending reputation

Don't rely on merely using different "From" addresses. The authenticated sending domain and underlying sending infrastructure matter for reputation. Klaviyo, for example, explicitly notes that branded sending domains let a brand build its own reputation rather than inheriting the reputation of a shared sending domain. Klaviyo Help Center

And I wouldn't automatically buy a dedicated IP for every client. A dedicated sending domain is usually the more important isolation mechanism; dedicated IPs make sense when a client's volume and sending pattern justify them.

My recommendation

If I were building the agency stack today:

• Mixed B2B/B2C clients → ActiveCampaign
• Mostly Shopify/ecommerce → Klaviyo
• Many clients + high-volume newsletters → Brevo
• Large enterprise clients already using CRM → HubSpot Enterprise

HubSpot is particularly interesting if you need actual infrastructure-level separation: its 2026 Enterprise functionality allows separate dedicated IPs to be associated with different brands, specifically to maintain separate sending reputations. HubSpot Knowledge Base

If you tell me roughly how many clients, average contacts per client, monthly sends, and whether they're mostly B2B or ecommerce, I can narrow this down to the best 2–3 setups and estimate what the agency would actually pay.

It also now supports separate branded sending domains for marketing, transactional and service email, which is useful when you want to isolate different types of traffic. Klaviyo

One nuance: a Klaviyo account can only have one active branded sending domain per send type, so for true client isolation I'd use separate Klaviyo accounts rather than trying to put many clients into one account. Klaviyo Help Center

klaviyo.com

💰 Brevo is interesting for a high-volume agency

Brevo is worth looking at if you're managing many clients and send volume matters more than sophisticated automation.

Its sub-organization model is specifically useful for separating clients while retaining centralized agency administration/billing. Mailsoftly

brevo.com

One thing I'd strongly recommend

If by "separate sending reputations" you mean:

Client A has a bad campaign → I don't want Client B's deliverability affected

then I'd structure your agency like this:

AGENCY
│
├── Client A
│   ├── Separate platform account
│   ├── clientA.com / email.clientA.com
│   ├── SPF + DKIM + DMARC
│   └── Client A sending reputation
│
├── Client B
│   ├── Separate platform account
│   ├── clientB.com / email.clientB.com
│   ├── SPF + DKIM + DMARC
│   └── Client B sending reputation
│
└── Client C
    ├── Separate platform account
    ├── clientC.com / email.clientC.com
    ├── SPF + DKIM + DMARC
    └── Client C sending reputation

Don't rely on merely using different "From" addresses. The authenticated sending domain and underlying sending infrastructure matter for reputation. Klaviyo, for example, explicitly notes that branded sending domains let a brand build its own reputation rather than inheriting the reputation of a shared sending domain.

And I wouldn't automatically buy a dedicated IP for every client. A dedicated sending domain is usually the more important isolation mechanism; dedicated IPs make sense when a client's volume and sending pattern justify them.

My recommendation

If I were building the agency stack today:

Yes. If your key requirement is one agency managing multiple clients while keeping each client’s sending reputation isolated, I’d narrow it to Brevo, Campaign Monitor, and MailerLite for traditional opt-in email marketing—and I’d look at dedicated cold-email infrastructure if you mean outbound prospecting.

My ranking

Brevo is probably my first choice if you're managing, say, 5–50 clients. Its current admin architecture supports sub-organizations, and Brevo explicitly supports assigning dedicated IPs to individual sub-organizations. Dedicated IPs are included with Enterprise. Brevo Help Brevo Help

Campaign Monitor is particularly attractive if your agency wants a clean client-management experience. Its agency offering is designed around managing client accounts, campaigns, automations, templates and reporting from one place. Campaign Monitor

MailerLite is surprisingly good operationally: one login can switch between completely separate client accounts, with each account maintaining its own subscribers, campaigns, automations and templates. MailerLite

The important part: don't confuse account separation with reputation separation

For what you're describing, I'd structure it roughly like:

Agency
→ Client A
→ Client B
→ Client C
→ Client D

And each client should have:

• Their own sending domain/subdomain
• Their own DKIM/SPF/DMARC
• Their own suppression/unsubscribe data
• Ideally their own dedicated IP or appropriately isolated IP pool at sufficient volume
• Separate bounce/complaint monitoring
• Separate reporting

That way, if Client A suddenly has a bad campaign or high complaint rate, you're not unnecessarily putting Client B's deliverability at risk.

One nuance: a dedicated IP isn't automatically better. At low volume, a dedicated IP can actually be harder to maintain because it needs enough consistent legitimate traffic and proper warming. The sending domain, authentication, list quality, engagement and complaint rate are hugely important too. The Open Rate Club

If you're doing cold email, that's a different answer

If by "email marketing" you actually mean cold outbound campaigns to prospects, I would not choose Brevo/Campaign Monitor/MailerLite as my first option.

I'd look at infrastructure specifically designed around per-client workspaces, domains, mailboxes and reputation isolation. For example, SendKit currently advertises dedicated IPs per client workspace and isolated infrastructure specifically for agencies. SendKit

So my recommendation would be:

• Newsletter / lifecycle / opt-in marketing: Brevo
• Agency managing lots of traditional email clients: Campaign Monitor
• Smaller agency / lower cost: MailerLite
• Complex CRM + automation: ActiveCampaign
• Shopify/ecommerce clients: Klaviyo
• Cold outbound: a dedicated outbound platform/infrastructure rather than a conventional ESP

If you tell me roughly how many clients you manage, monthly sends per client, and whether this is opt-in marketing or cold email, I can give you a much more specific recommendation—including the account/IP/domain architecture I'd use.

If by "email marketing" you actually mean cold outbound campaigns to prospects, I would not choose Brevo/Campaign Monitor/MailerLite as my first option.

I'd look at infrastructure specifically designed around per-client workspaces, domains, mailboxes and reputation isolation. For example, SendKit currently advertises dedicated IPs per client workspace and isolated infrastructure specifically for agencies.

So my recommendation would be:

The important part: don't confuse account separation with reputation separation

For what you're describing, I'd structure it roughly like:

Agency
→ Client A
→ Client B
→ Client C
→ Client D

And each client should have:

Yes. If your main requirement is that each client has an independent sending reputation, I would prioritize platforms with true separate accounts/subaccounts, rather than putting all clients into one account and merely segmenting them.

My shortlist

1. Campaign Monitor — probably the cleanest agency setup

Campaign Monitor is particularly interesting for your use case because its agency offering is built around managing separate client accounts from one place. Campaign Monitor specifically positions its agency product around managing client accounts, campaigns, automations, templates and reporting. Campaign Monitor

This is the one I'd investigate first if you're managing lots of conventional SMB/local-business clients.

Why I like it:

• Separate client accounts
• Separate client data/lists
• Separate sending domains
• Agency-level management
• Client-specific reporting
• Less risk of accidentally mixing clients

Independent agency-focused comparisons also rank Campaign Monitor highly specifically for multi-client management. Hack'celeration

2. Klaviyo — my pick for e-commerce agencies

Klaviyo has a very good architecture for this. You can manage multiple accounts through a single login/portfolio, while the underlying accounts remain distinct. Klaviyo explicitly supports portfolios containing multiple accounts and separate permissions. Klaviyo Academy Klaviyo Help Center

More importantly for your concern, Klaviyo supports branded sending domains, and different sending streams can have their own reputation. Klaviyo Help Center

I'd choose Klaviyo if your agency works heavily with:

• Shopify
• DTC/e-commerce
• Customer lifecycle automation
• Abandoned cart
• Post-purchase
• Revenue attribution
• Sophisticated segmentation

3. ActiveCampaign — excellent for B2B/automation agencies

ActiveCampaign is another strong choice if you're selling automation rather than simply newsletter management.

Its big advantage is the automation/CRM side. Agency-focused comparisons consistently put it near the top for agencies doing sophisticated workflows. Sasanova PG Systems Lab

I'd lean toward it for clients needing things like:

Lead → nurture → scoring → sales notification → CRM → re-engagement

rather than just:

Newsletter → promotional campaign

4. Brevo — strong value option

Brevo is worth considering if you have a lot of smaller clients and email volume varies substantially between them. Its pricing model can make more sense than contact-based pricing when you're managing many client lists. Agency-focused comparisons also highlight its sub-organization model. Sasanova

The tradeoff is that its agency ecosystem and automation depth aren't as compelling as the top choices.

5. HubSpot — when clients are enterprise

HubSpot can actually provide very strong separation, but you're moving into a much more expensive/complex ecosystem.

HubSpot supports multiple organizations/accounts with centralized management while keeping organizational data separate. HubSpot It also now documents separate dedicated IPs per brand, specifically to maintain separate sending reputations. HubSpot Knowledge Base

That's attractive when you're dealing with larger clients where deliverability, CRM, permissions, compliance and reporting all matter.

The architecture I'd recommend

If you're building an agency from scratch, I'd structure it like this:

YOUR AGENCY
│
├── Client A
│   ├── Client-owned account
│   ├── clientA.com sending domain
│   └── Client A reputation
│
├── Client B
│   ├── Client-owned account
│   ├── clientB.com sending domain
│   └── Client B reputation
│
├── Client C
│   ├── Client-owned account
│   ├── clientC.com sending domain
│   └── Client C reputation
│
└── Client D
    ├── Client-owned account
    ├── clientD.com sending domain
    └── Client D reputation

That's preferable to:

ONE AGENCY ACCOUNT
    ↓
100 client lists
    ↓
100 sending domains

Even if the latter technically allows domain separation, I prefer account-level isolation for an agency. It makes permissions, billing, data ownership, reporting, offboarding, integrations and deliverability much cleaner.

One important distinction

If by "sending reputation" you mean marketing/newsletter email, I'd shortlist:

Klaviyo / Campaign Monitor / ActiveCampaign

If you mean cold outbound sales email, that's a different category entirely. I'd use a dedicated outbound platform and generally give each client separate sending domains/mailboxes rather than putting cold outreach and opt-in marketing mail on the same infrastructure.

If you tell me roughly how many clients you manage (e.g. 10, 50, 200), monthly sends per client, and whether they're mostly e-commerce or B2B, I can narrow this down to the best 2–3 platforms and explain the account/reputation architecture I'd use.

Yes. If your key requirement is one agency managing multiple clients while keeping each client’s sending reputation isolated, I’d prioritize platforms with true client accounts/sub-accounts + client-specific sending domains, rather than putting everyone into one shared account.

My shortlist

🥇 Campaign Monitor — probably the closest match

Campaign Monitor is particularly interesting for an agency because its agency setup is designed around separate client accounts managed from a central agency relationship. Its agency offering emphasizes managing client accounts, campaigns, automations, templates and reporting in one place. Campaign Monitor Hack'celeration

That matters because you don't want:

Agency account → Client A list + Client B list + Client C list

You want:

Agency
→ Client A account → Client A domain/reputation
→ Client B account → Client B domain/reputation
→ Client C account → Client C domain/reputation

That's a much cleaner operational model.

campaignmonitor.com

🥈 ActiveCampaign — better if automation is the product

ActiveCampaign is my pick if your agency is selling CRM + lifecycle automation + lead nurturing, rather than primarily newsletters.

Its agency program is built around managing/reselling separate client accounts, and it has strong automation capabilities. PG Systems Lab Sasanova

For reputation isolation, ActiveCampaign specifically recommends using sending subdomains to separate different email streams and build more controlled sender reputations. ActiveCampaign Help Center

For example:

• email.clientA.com
• email.clientB.com
• email.clientC.com

Each client should ideally have its own domain/subdomain, authentication, list, account and reputation.

🥉 Klaviyo — excellent for ecommerce agencies

Klaviyo is excellent if your clients are Shopify/ecommerce brands.

Klaviyo supports branded sending domains, and explicitly explains that using a branded domain gives you control over the domain's sender reputation rather than relying on a shared sending domain. Klaviyo Help Center Klaviyo Help Center

The catch is that I wouldn't choose Klaviyo just because you're an agency. I'd choose it because your clients need Klaviyo's ecommerce-specific functionality.

help.klaviyo.com

One important distinction: domain reputation vs IP reputation

If by "separate sending reputations" you mean you absolutely don't want Client A's bad sending behavior to affect Client B, don't rely solely on separate lists inside one account.

I'd structure it roughly as:

Client isolation

• Separate client account/sub-account
• Separate client list/database
• Separate sending domain or subdomain
• Separate DKIM/SPF/DMARC configuration
• Separate unsubscribe/suppression data
• Ideally separate IP infrastructure for sufficiently large senders

A branded sending domain is particularly important because mailbox providers evaluate domain reputation; Klaviyo, for example, explicitly notes that a branded domain means the client is the only sender affecting that domain's reputation. Klaviyo Help Center

What I'd choose

If you're building an agency that might manage 10–50+ client email programs, I'd narrow it to:

Campaign Monitor → best if your priority is agency operations + client isolation + straightforward campaign management.

ActiveCampaign → best if you're selling sophisticated automation/CRM.

Klaviyo → best if most clients are ecommerce.

Brevo → worth considering if cost per client/send volume is a major concern; its sub-organization model can work well for agencies. Sasanova

I would not build a multi-client agency around one giant Mailchimp account simply by separating clients with tags/lists. The account architecture and reputation isolation aren't as clean for the use case you're describing.

If you tell me roughly how many clients you manage, monthly sends per client, and whether this is newsletter marketing or cold/outbound email, I can narrow this down to the best 2–3 options and explain the exact account/domain setup I'd use.

For an agency managing multiple client email programs with separate sending reputations, I would avoid the typical “one big account with lists separated by tags” approach. You want true client isolation: separate domains, DKIM/SPF/DMARC, contact databases, suppression lists, reporting, and ideally sub-accounts. InboxAlly Mailpro

The best fit depends on the type of clients you manage:

Best overall agency options

Hack'celeration Sasanova## My shortlist by agency model

1. Lead generation / B2B agency

Choose ActiveCampaign.

Good fit if you build:

• Lead nurture sequences
• Sales funnels
• Webinar campaigns
• CRM-connected automations
• Lifecycle marketing

You can give each client their own account/workspace rather than mixing senders. Sasanova

2. Email production agency (newsletters, campaigns, design)

Choose Campaign Monitor.

Good fit if your service is:

• Monthly newsletters
• Promotional campaigns
• Client approvals
• Multiple brands managed by account managers

Its agency-oriented client management is one of its stronger points. Hack'celeration

3. Lots of smaller clients with modest volume

Choose Brevo.

Good fit if you have:

• 10–100 SMB clients
• Lower sending volume
• Need centralized billing
• Need separate client workspaces

Sasanova### 4. Shopify/ecommerce clients

Usually let each client run their own Klaviyo account and give your agency access.

Why:

• Their data model is built around each store/customer lifecycle.
• Mixing multiple stores into one agency account is usually a bad operational pattern.

Deliverability setup I would require regardless of platform

For each client:

1. Use the client’s sending domain

• Example: mail.clientdomain.com
• Configure:

• SPF
• DKIM
• DMARC
• Keep separate:

• Subscriber lists
• Bounce/suppression data
• Templates
• Sending reputation
• Avoid:

• One agency-owned sending domain for all clients
• Shared lists
• Shared IPs for very different senders

A bad sender should not damage your other clients. InboxAlly Mailpro

If I were building an agency stack today

• 5–20 B2B clients: ActiveCampaign
• 20+ newsletter clients: Campaign Monitor or Brevo
• Ecommerce portfolio: Klaviyo per client
• Enterprise retainers: HubSpot per client

One important question changes the answer: are these clients mostly B2B lead generation, ecommerce, or newsletter/content marketing? Also, roughly how many client accounts are you managing (5, 20, 100+)?

If your main requirement is an agency managing multiple clients while keeping each client's sending reputation isolated, I’d narrow it to Campaign Monitor, HighLevel, ActiveCampaign, and Klaviyo, with an important distinction between account isolation and domain/IP reputation isolation.

My ranking

🥇 Campaign Monitor — best pure agency setup

This is probably the cleanest fit if you're primarily an email marketing agency.

Campaign Monitor specifically supports agencies managing separate client accounts from one place, with each client's account kept separate. Campaign Monitor

The big advantage is that you're not trying to simulate client separation using tags, lists, or folders. Each client is an actual isolated account.

I'd choose it if you have something like:

Agency → 20 clients → each client has its own contacts, domain, campaigns, reporting and reputation.

Its weakness is automation/CRM depth compared with ActiveCampaign or Klaviyo.

campaignmonitor.com

🥈 HighLevel — best if you're selling lead generation

If your agency does funnels + CRM + automation + email/SMS, I'd seriously consider HighLevel.

Its architecture is naturally agency → sub-account/client, and it supports dedicated sending domains at the sub-account level. It also lets you assign different domains to different types of email traffic. HighLevel Support Portal

That's attractive because you can have:

• Client A → send.clientA.com
• Client B → send.clientB.com
• Client C → send.clientC.com

rather than putting everybody's email infrastructure under one sender.

HighLevel also supports dedicated IPs, including the ability to rebill the IP cost to sub-accounts. HighLevel Support Portal

For a performance/lead-gen agency, this would probably be my #1.

gohighlevel.com

🥉 ActiveCampaign — best automation engine

If your clients need sophisticated lifecycle automation, ActiveCampaign is excellent.

You can use separate sending subdomains to help isolate different mail streams and build distinct sender reputations. ActiveCampaign itself recommends subdomains as a way to differentiate email traffic and manage reputation. ActiveCampaign Help Center

The catch is that ActiveCampaign isn't as elegant as Campaign Monitor/HighLevel for agency-wide multi-client administration. You generally end up managing separate client accounts rather than having everything feel like true subaccounts.

So I'd pick it when:

automation sophistication > agency administration convenience.

activecampaign.com

Klaviyo — excellent, but mostly for e-commerce

For Shopify/e-commerce clients, I'd actually put Klaviyo above the others.

You can configure branded sending domains and separate marketing/transactional/service sending streams. Klaviyo Help Center Klaviyo Help Center

However, there's an important nuance for your requirement:

Don't assume that putting clients in separate Klaviyo accounts automatically means completely independent reputation.

Klaviyo says a branded sending domain can be used across multiple accounts, but the performance of those accounts affects the reputation of that domain. Klaviyo Help Center

So for an agency, I'd give every client their own sending domain/subdomain rather than sharing one.

klaviyo.com

The architecture I'd recommend

Regardless of platform, I'd structure it like this:

YOUR AGENCY
│
├── Client A
│   ├── Client-owned sending domain
│   ├── Client-owned DKIM/SPF/DMARC
│   ├── Client-specific tracking domain
│   ├── Client-specific IP (if volume warrants it)
│   └── Client-specific suppression/list data
│
├── Client B
│   ├── Client-owned sending domain
│   ├── Client-owned DKIM/SPF/DMARC
│   ├── Client-specific tracking domain
│   ├── Client-specific IP (if volume warrants it)
│   └── Client-specific suppression/list data
│
└── Client C
    └── Same isolation

The sending domain is more important than simply having separate "accounts."

For example, if you have 15 clients but all 15 send through the same agency-owned domain/IP infrastructure, you haven't achieved the reputation isolation you're looking for.

Also, don't interpret "separate sending reputation" as meaning that subdomains make you immune from problems with the parent domain. ActiveCampaign explicitly notes that subdomain reputation can still affect the base domain, albeit less strongly. ActiveCampaign Help Center

My recommendation by agency type

• Pure email marketing agency: Campaign Monitor
• Lead-gen / marketing automation agency: HighLevel
• B2B / complex automation: ActiveCampaign
• E-commerce agency: Klaviyo
• Very cost-sensitive / lots of smaller clients: Brevo

One other option worth considering is Mailchimp Transactional, particularly if you're building your own agency platform/API layer: its subaccounts explicitly allow different clients/senders to have separate reputation, activity, reports and quotas under one account. mailchimp.com

If you tell me roughly how many clients you manage (e.g. 10, 50, 200), monthly sends per client, and whether they're B2B or e-commerce, I can narrow this down to the best 2–3 architectures and explain how I'd structure the domains/IPs.

If your clients need sophisticated lifecycle automation, ActiveCampaign is excellent.

You can use separate sending subdomains to help isolate different mail streams and build distinct sender reputations. ActiveCampaign itself recommends subdomains as a way to differentiate email traffic and manage reputation. ActiveCampaign Help Center

The catch is that ActiveCampaign isn't as elegant as Campaign Monitor/HighLevel for agency-wide multi-client administration. You generally end up managing separate client accounts rather than having everything feel like true subaccounts.

So I'd pick it when:

automation sophistication > agency administration convenience.

activecampaign.com

Klaviyo — excellent, but mostly for e-commerce

For Shopify/e-commerce clients, I'd actually put Klaviyo above the others.

You can configure branded sending domains and separate marketing/transactional/service sending streams. Klaviyo Help Center Klaviyo Help Center

However, there's an important nuance for your requirement:

Don't assume that putting clients in separate Klaviyo accounts automatically means completely independent reputation.

Klaviyo says a branded sending domain can be used across multiple accounts, but the performance of those accounts affects the reputation of that domain. Klaviyo Help Center

So for an agency, I'd give every client their own sending domain/subdomain rather than sharing one.

klaviyo.com

The architecture I'd recommend

Regardless of platform, I'd structure it like this:

YOUR AGENCY
│
├── Client A
│   ├── Client-owned sending domain
│   ├── Client-owned DKIM/SPF/DMARC
│   ├── Client-specific tracking domain
│   ├── Client-specific IP (if volume warrants it)
│   └── Client-specific suppression/list data
│
├── Client B
│   ├── Client-owned sending domain
│   ├── Client-owned DKIM/SPF/DMARC
│   ├── Client-specific tracking domain
│   ├── Client-specific IP (if volume warrants it)
│   └── Client-specific suppression/list data
│
└── Client C
    └── Same isolation

The sending domain is more important than simply having separate "accounts."

For example, if you have 15 clients but all 15 send through the same agency-owned domain/IP infrastructure, you haven't achieved the reputation isolation you're looking for.

Also, don't interpret "separate sending reputation" as meaning that subdomains make you immune from problems with the parent domain. ActiveCampaign explicitly notes that subdomain reputation can still affect the base domain, albeit less strongly.

My recommendation by agency type

For an agency managing multiple client email programs, the biggest factor is not the email editor — it is account isolation, deliverability controls, permissions, and client handoff. You generally want each client to have:

• Separate sending domains (e.g., news.clientA.com vs. news.clientB.com)
• Separate SPF/DKIM/DMARC authentication
• Separate suppression lists
• Separate analytics/reporting
• Ideally separate sending reputation (dedicated IPs or isolated sending pools for higher-volume clients)

Putting all clients into one shared account can create reputation spillover if one client has poor list hygiene or spam complaints. InboxAlly Mailpro

Best platforms for agencies

1. ActiveCampaign — best for automation-heavy agencies

Best fit: B2B, SaaS, lead nurture, complex funnels

Pros:

• Very strong automation builder
• CRM + email automation together
• Good team permissions
• Agency/reseller options
• Works well when each client needs sophisticated journeys Sasanova

Cons:

• Contact-based pricing can get expensive across many clients
• Less focused on pure agency white-label workflows than some competitors

Choose it if: your agency sells strategy + automation builds, not just newsletters.

2. Campaign Monitor — best pure agency workflow

Best fit: agencies running many brands/newsletters

Pros:

• Designed around managing multiple client accounts
• Client-level separation
• Agency-friendly permissions and reporting
• Strong for recurring newsletter production workflows Campaign Monitor

Cons:

• Automation is not as deep as ActiveCampaign
• Can become expensive at scale

Choose it if: your agency manages many clients that need reliable campaign execution and reporting.

3. Brevo — best cost efficiency

Best fit: many smaller clients with moderate sending volume

Pros:

• Send-based pricing can work well for agencies
• Sub-organization/account structures
• Includes SMS and CRM features
• Lower cost for clients with large lists but fewer sends Sasanova

Cons:

• Less sophisticated automation
• Agency workflow is not as polished as dedicated agency tools

Choose it if: you manage lots of SMB clients and margins matter.

4. Klaviyo — best for e-commerce clients

Best fit: Shopify/DTC brands

Pros:

• Excellent segmentation
• Strong revenue attribution
• Great ecommerce automation

Cons:

• Usually each client needs their own account
• Less of an agency command-center model

Choose it if: most clients are ecommerce brands.

5. Mailjet — good for transactional + agency sending

Best fit: agencies mixing marketing and transactional email

Pros:

• Supports client separation through subaccounts
• Designed for agencies managing multiple clients
• Collaboration and permissions features Sinch Mailjet

Cons:

• Marketing automation depth is lighter than ActiveCampaign/Klaviyo

My typical agency stack recommendation

Small agency (5–15 clients)

• Campaign Monitor for newsletters/campaign management
• ActiveCampaign for clients paying for automation
• Separate client sending domains

Growth agency (15–50 clients)

• Use a platform with true subaccounts/workspaces
• Standardize onboarding: - client-owned domain
• DNS authentication checklist
• list hygiene rules
• reporting template
• approval workflow

Ecommerce agency

• Let each client own their Klaviyo account
• Agency gets partner/admin access
• Avoid combining stores into one account

Deliverability setup I would require regardless of platform

For every client:

1. Client-owned sending domain/subdomain

Example: email.brand.com

2. SPF + DKIM + DMARC configured
3. Dedicated IP only when volume justifies it

(a low-volume client usually benefits more from a reputable shared pool)

4. Separate lists and suppression management
5. Client-level monitoring of:

• bounce rate
• complaint rate
• engagement decay
• spam placement

If your agency is primarily newsletter production, I’d lean Campaign Monitor. If it is funnels, lifecycle marketing, and automation, I’d lean ActiveCampaign. If it is many small clients where margin matters, I’d look at Brevo. Hack'celeration Sasanova

I can also compare these specifically for 10–100 client accounts with white-label reporting and separate sender reputation, which changes the ranking quite a bit.

Yes. If your priority is one agency managing many clients while keeping each client’s sending reputation isolated, I’d narrow it to HighLevel, Campaign Monitor, and Klaviyo—with different winners depending on your client mix.

My ranking

1. HighLevel — probably the best agency architecture

If you're running, say, 10–50 client accounts, I'd seriously consider gohighlevel.com.

Its sub-account model maps very naturally to an agency: Client A, Client B, Client C, etc. More importantly for your question, HighLevel's current LC Email system lets each sub-account use its own dedicated sending domain. HighLevel explicitly recommends not sharing sending domains between sub-accounts because of cross-account reputation risk. HighLevel Support Portal HighLevel Support Portal

For example:

• Client A → send.clienta.com
• Client B → send.clientb.com
• Client C → mail.clientc.com

Each can then have its own authentication and reputation.

It also lets you assign different domains to different types of mail—campaigns, workflows, 1:1, etc.—which is useful if you're serious about deliverability. HighLevel Support Portal

Big advantage: you get CRM, funnels, SMS, automation, calendars, etc. alongside email.

Downside: it's considerably more of an all-in-one agency platform than a pure email platform, so the email experience isn't as elegant as Klaviyo or Campaign Monitor.

2. Campaign Monitor — excellent if you're primarily an email agency

campaignmonitor.com is particularly interesting for your use case because its agency offering is specifically designed around managing separate client accounts from one agency environment. Campaign Monitor says each client account stays separate while the agency can manage campaigns, automations, templates and reporting centrally. Campaign Monitor

This is the one I'd look at if your agency's business is essentially:

"We manage email marketing for 20 different companies."

rather than:

"We build complete marketing/CRM systems for 20 companies."

A 2026 agency-focused comparison also ranked Campaign Monitor strongest for multi-client account management and isolated client sending domains. Hack'celeration

3. Klaviyo — best when your clients are Shopify/e-commerce

klaviyo.com is my choice if most of your clients are e-commerce brands.

Its deliverability infrastructure is excellent, and you can configure branded sending domains for different types of traffic. Klaviyo Help Center Klaviyo Help Center

However, there's an important distinction for an agency:

Separate Klaviyo accounts ≠ automatically isolated reputation if you reuse the same sending domain.

Klaviyo explicitly says that if the same branded sending domain is used across multiple Klaviyo accounts, the performance of those accounts can affect that domain's reputation. Klaviyo Help Center

So I'd structure it as:

• Client A → separate Klaviyo account + Client A sending domain
• Client B → separate Klaviyo account + Client B sending domain
• Client C → separate Klaviyo account + Client C sending domain

That gives you much cleaner isolation.

The architecture I'd recommend

If deliverability isolation is your #1 requirement, don't just look for "separate accounts."

You want four layers of separation:

Client account → client sending domain/subdomain → client authentication → ideally client sending infrastructure

For example:

YOUR AGENCY
│
├── Client A
│   ├── Platform sub-account
│   ├── send.clientA.com
│   ├── DKIM/SPF/DMARC
│   └── Client A reputation
│
├── Client B
│   ├── Platform sub-account
│   ├── send.clientB.com
│   ├── DKIM/SPF/DMARC
│   └── Client B reputation
│
└── Client C
    ├── Platform sub-account
    ├── send.clientC.com
    ├── DKIM/SPF/DMARC
    └── Client C reputation

And I would not put all your clients behind something like:

send.youragency.com

if your objective is genuine reputation isolation.

HighLevel's current documentation makes this exact point: each sub-account should have its own dedicated sending domain rather than sharing one across clients. HighLevel Support Portal

My recommendation

• General-purpose marketing agency: HighLevel
• Pure email marketing agency: Campaign Monitor
• E-commerce agency: Klaviyo
• B2B + sophisticated automation: ActiveCampaign
• Maximum deliverability isolation: choose the platform based on the above, but give each client its own authenticated sending domain regardless of platform.

If you tell me roughly how many client accounts you manage, monthly sends per client, and whether they're mostly B2B, local businesses, or e-commerce, I can narrow this down to the best 2–3 setups and what the agency-level costs would look like.

If deliverability isolation is your #1 requirement, don't just look for "separate accounts."

You want four layers of separation:

Client account → client sending domain/subdomain → client authentication → ideally client sending infrastructure

For example:

YOUR AGENCY
│
├── Client A
│   ├── Platform sub-account
│   ├── send.clientA.com
│   ├── DKIM/SPF/DMARC
│   └── Client A reputation
│
├── Client B
│   ├── Platform sub-account
│   ├── send.clientB.com
│   ├── DKIM/SPF/DMARC
│   └── Client B reputation
│
└── Client C
    ├── Platform sub-account
    ├── send.clientC.com
    ├── DKIM/SPF/DMARC
    └── Client C reputation

And I would not put all your clients behind something like:

send.youragency.com

if your objective is genuine reputation isolation.

HighLevel's current documentation makes this exact point: each sub-account should have its own dedicated sending domain rather than sharing one across clients.

My recommendation

Yes. If your key requirement is an agency managing multiple clients where each client needs an independently protected sending reputation, I’d look beyond the typical all-in-one marketing platforms.

My shortlist

1. Mailgun — probably my first choice

mailgun.com

Mailgun has subaccounts specifically designed for this kind of multi-tenant setup. Each subaccount can have its own sending domains, IPs, API keys, users, and reporting while remaining under your agency's primary account. Mailgun Documentation Mailgun Documentation

That maps very nicely to:

Agency → Client A subaccount → Client A domains/IPs
Agency → Client B subaccount → Client B domains/IPs
Agency → Client C subaccount → Client C domains/IPs

It also lets you separate marketing vs. transactional sending with different domains/subdomains, which is important for reputation management. Mailgun Documentation Mailgun Documentation

I'd choose Mailgun if you want: relatively easy infrastructure, strong API capabilities, client-level separation, and an agency-friendly operational model.

2. Amazon SES — best if you're technically capable

aws.amazon.com

SES has become particularly interesting for agencies because its newer tenant management functionality is explicitly designed for organizations sending on behalf of multiple downstream customers.

A tenant can have its own identities, configuration sets, templates, and reputation metrics. AWS Documentation

You can additionally assign dedicated IP pools to different tenants/configuration sets, giving you genuine IP-level reputation isolation. AWS Documentation AWS Documentation

For example:

• Client A → Tenant A → IP Pool A
• Client B → Tenant B → IP Pool B
• Client C → Tenant C → IP Pool C

SES also offers managed dedicated IPs, which automatically handles IP allocation, scaling, and warmup. AWS Documentation

The downside is that SES is infrastructure, not really an agency-friendly marketing application. You'd likely need another layer for campaign creation, lists, automation, analytics, etc.

One important distinction

If by "separate sending reputations" you mean truly independent reputation, don't just create separate workspaces/accounts inside a marketing SaaS and assume the reputations are isolated.

There are at least two reputation components:

• Domain reputation — your client's sending domain/subdomain
• IP reputation — the IP infrastructure sending the mail

Mailgun explicitly notes that reputation is tied to both the IP and the domain. Mailgun Documentation

For a serious agency setup, I'd generally give each client:

Client-specific sending domain/subdomain + DKIM/SPF/DMARC + isolated sending infrastructure where volume justifies it.

Also, don't automatically put every client on a dedicated IP. Dedicated IPs need sufficient, consistent volume to develop and maintain reputation; AWS specifically recommends shared IPs for low-volume or unpredictable senders. AWS Documentation

What I'd build for an agency

If you're managing, say, 10–100 clients, I'd lean toward:

Marketing UI/automation layer → Mailgun → client-specific subaccounts/domains/IP pools

If you're building your own SaaS/platform and want to send enormous volumes on behalf of clients, I'd seriously consider:

Your application → Amazon SES tenant management → per-client identities/configuration sets/IP pools

If you tell me roughly how many clients you have, emails/month per client, and whether you need a visual campaign builder/automation, I can narrow this down to the best 2–3 architectures and estimate the costs.

For an agency, the “best” email marketing platform is usually not the one with the best campaign editor — it’s the one that lets you separate clients operationally and protect each client’s sender reputation. You generally want:

• Separate client workspaces/sub-accounts
• Separate sending domains (and ideally isolated IP strategy for large senders)
• Client-level permissions
• Consolidated billing
• Agency reporting/white-label options
• Ability to move a client out cleanly if they leave www.mailpro.com

My shortlist:

hackceleration.com

My practical recommendations by agency type

1. You manage 5–50 clients with different industries
→ ActiveCampaign or Brevo

A common setup:

• Agency master account
• One client workspace/account per client
• Client-owned sending domain
• Client-specific SPF/DKIM/DMARC
• Client-specific reporting

2. You mainly do newsletters/content marketing
→ Campaign Monitor

It fits agencies where the workflow is:
client → approval → scheduled campaigns → monthly reporting. hackceleration.com

3. You do retention/ecommerce email
→ Klaviyo

Especially if clients are Shopify/DTC brands. You’ll usually get better revenue attribution and segmentation than general-purpose tools.

4. You sell “we manage all your marketing”
→ HubSpot

More expensive, but the client relationship, CRM, sales pipeline, attribution, and reporting are stronger.

Platforms I would be cautious with for an agency

• One giant shared account with all clients mixed together — risky. A bad list or spam complaint from one client can damage others. www.mailpro.com
• Using your agency domain as the sender — clients should send from their own authenticated domains.
• Letting every client use the same IP pool without controls — high-volume or risky senders should be isolated. www.inboxeagle.com

If I were building an agency email stack today

For most agencies:

1. ActiveCampaign for automation clients
2. Klaviyo for ecommerce clients
3. Campaign Monitor for newsletter clients
4. A separate deliverability monitoring process for every client domain

The platform matters, but the architecture matters more: every client should have their own sending identity and reputation boundary. mailflowauthority.com

If you tell me:

• number of clients you manage,
• monthly send volume,
• B2B vs ecommerce vs newsletters,
• whether clients need login access,

I can narrow this to 1–2 best choices.

Yes. If your agency is managing multiple clients and you specifically want each client to have an independent sending reputation, I’d prioritize true account/sub-account separation over simply having multiple lists inside one account.

My shortlist

🥇 My default choice: ActiveCampaign

For a general marketing agency, I'd probably choose ActiveCampaign.

Its agency model is designed around managing separate client accounts, rather than putting all clients into one giant account. That matters because you can keep each client's contacts, automations, permissions, billing, and sending infrastructure separated. Current agency comparisons also put ActiveCampaign near the top for agencies needing sophisticated automation and multiple client accounts. www.sasanova.com

I'd structure it like:

Agency
→ Client A account → email.clientA.com
→ Client B account → email.clientB.com
→ Client C account → email.clientC.com

That is much safer than:

One agency account
→ Client A list
→ Client B list
→ Client C list

because you don't want one client's poor engagement or spam complaints affecting the others.

🛒 If your clients are mostly e-commerce: Klaviyo

I'd choose Klaviyo instead if you're primarily doing Shopify/DTC retention marketing.

Klaviyo supports branded sending domains, and its documentation specifically notes that a branded domain lets you build reputation on your own domain rather than relying on a shared Klaviyo sending domain. help.klaviyo.com

One important nuance: Klaviyo says a branded sending domain can technically be used by multiple Klaviyo accounts, but the performance of all those accounts affects that domain's reputation. So for your use case, I'd give each client its own sending subdomain/domain, rather than sharing one across clients. help.klaviyo.com

🏢 If you want an agency operating system: HighLevel

If you're not just selling email but also CRM + funnels + SMS + automation + lead management, HighLevel becomes very compelling. Its architecture is built around agency/client sub-accounts.

I'd pick it when the agency is essentially selling a marketing system rather than an email-marketing service.

💰 Brevo is worth considering

Brevo is interesting if you have clients with large databases and lots of sends, because its pricing model can be more attractive than contact-based platforms. It also has sub-organization functionality aimed at agencies. www.sasanova.com

The bigger issue: domain reputation vs. IP reputation

This is the part I'd be especially careful about.

If by "separate sending reputations" you mean:

"If Client A starts getting terrible engagement/spam complaints, I don't want Client B's deliverability to suffer."

Then I'd implement both account and domain separation.

For example:

• Client A → send.clienta.com
• Client B → send.clientb.com
• Client C → send.clientc.com

And each gets its own:

• SPF
• DKIM
• DMARC alignment
• sending domain/subdomain
• audience/list
• suppression list
• sending account
• reputation monitoring

You don't necessarily need a dedicated IP for every small client. Domain reputation is extremely important, and a dedicated IP can actually be counterproductive for low-volume senders because there's insufficient volume to establish a good IP reputation.

Klaviyo's documentation, for example, explicitly distinguishes domain and IP reputation and recommends branded sending domains to give the sender more control over reputation. help.klaviyo.com

One other option: keep clients' accounts in their ownership

For a serious agency, I actually like this model:

Client owns the platform account → agency gets partner/admin access.

That makes offboarding painless. The client keeps:

• their account
• their contacts
• their sending history
• their domain configuration
• their reputation
• their billing

The agency is the operator, not the owner.

Mailchimp, for example, explicitly supports agencies connecting to and switching between client-owned accounts through its agency program. mailchimp.com

My recommendation by agency type

General-purpose agency:
→ ActiveCampaign

Shopify/e-commerce agency:
→ Klaviyo

Lead-gen agency selling CRM + automation + SMS:
→ HighLevel

Lots of clients + high sending volume + cost sensitivity:
→ Brevo

Enterprise/B2B clients already invested in CRM:
→ HubSpot — it now has multi-account management designed specifically to keep separate HubSpot accounts while providing centralized oversight. www.hubspot.com

If you tell me roughly how many clients you manage, average contacts/client, monthly emails/client, and whether they're mostly B2B or e-commerce, I can narrow this down to the 2 best platforms and show you how I'd architect the accounts/domains/IPs for maximum reputation isolation.

For an agency managing multiple client email programs with separate sending reputations, I would prioritize platforms that support:

• True client/workspace separation
• Separate sending domains (and ideally IP options)
• Role-based access for your team and clients
• White-label reporting
• Centralized billing or agency management
• Template/workflow reuse without mixing data

A shared account with multiple lists is usually a bad idea because one client's poor list hygiene or spam complaints can hurt other clients' deliverability. Each client should ideally have their own sending domain/subdomain, DKIM/SPF/DMARC setup, suppression lists, and reputation history. ustechautomations.comwww.inboxeagle.com

Best options by agency type

My shortlist

1. Campaign Monitor — best overall agency email platform

• Best fit if you manage many brands and mostly deliver newsletters, campaigns, and automations.
• Designed around client separation and agency workflows. www.campaignmonitor.com
• Good when clients want reporting and you want operational simplicity.

2. ActiveCampaign — best if your agency sells automation
Choose this if your service includes:

• lead nurturing
• sales funnels
• CRM workflows
• behavioral triggers
• complex segmentation

It is more powerful than Campaign Monitor, but you may spend more time managing each client's automation architecture. hackceleration.com

3. Klaviyo — best for ecommerce retainers
For Shopify brands, I would often pick Klaviyo over general ESPs because the revenue attribution and customer-event data are usually more valuable than simple campaign management.

Recommended agency architecture

A scalable setup usually looks like:

Client A

• email.clienta.com
• Client A DKIM/SPF/DMARC
• Client A suppression list
• Client A analytics

Client B

• email.clientb.com
• Client B authentication
• Client B reputation

Agency:

• Master admin
• Shared templates
• Standard operating procedures
• Client reporting dashboard

Avoid these setups

❌ One agency domain sending for every client
Example: news@youragency.com sending campaigns for 20 brands

❌ One ESP account with 20 unrelated client lists

❌ Shared SMTP credentials across clients

Those approaches create reputation spillover risk. ustechautomations.comwww.inboxeagle.com

My recommendation by agency size

• 1–5 clients: ActiveCampaign or Klaviyo accounts per client
• 5–25 clients: Campaign Monitor agency setup or ActiveCampaign partner-style management
• 25+ clients: Consider a multi-tenant platform/workspace model or dedicated ESP infrastructure

If you tell me:

1. number of clients you manage,
2. monthly email volume,
3. B2B vs ecommerce, and
4. whether clients need login access,

I can narrow this down to the best stack.

For an agency, the key requirement is not just email features — it is client isolation. You want each client to have:

• Separate sending domains/subdomains (e.g., mail.clientA.com vs. mail.clientB.com)
• Separate DKIM/SPF/DMARC authentication
• Separate suppression lists and subscriber databases
• Separate reporting
• Ideally separate IP reputation or at least isolated sending pools
• Agency-level access, permissions, and billing control

Avoid putting all clients into one shared account/sending identity. One client's bad list hygiene or spam complaints can hurt everyone. ustechautomations.com

Best platforms for agencies

My typical recommendations by agency type

1. Full-service marketing agency (5–50 clients)
→ Campaign Monitor or ActiveCampaign

• Campaign Monitor is simpler for newsletters, campaigns, approvals, and client reporting. www.campaignmonitor.com
• ActiveCampaign is better if you are building complex automations, lead scoring, and CRM workflows. www.sasanova.com

2. E-commerce agency
→ Klaviyo

Each store should have its own Klaviyo account and sending domain. Don't pool multiple brands together.

3. Lead generation / cold outreach agency
→ Consider a dedicated sending stack rather than a traditional ESP:

• Separate domains per client
• Separate mailboxes
• Warm-up process
• Tools like Amazon Web Services SES, SendGrid, Mailgun, or Postmark for infrastructure

4. Agency selling email as a white-label service
Look for:

• Subaccounts/workspaces
• Client permissions
• White-label reporting
• Reseller billing

Architecture I would use for a serious agency

Agency master account
        |
        +-- Client A workspace
        |       |
        |       +-- clientA.com sending domain
        |       +-- DKIM/SPF/DMARC
        |       +-- client A lists + automations
        |
        +-- Client B workspace
                |
                +-- clientB.com sending domain
                +-- DKIM/SPF/DMARC
                +-- client B lists + automations

My shortlist

If I were choosing today:

1. ActiveCampaign — best overall for agencies doing real automation
2. Campaign Monitor — best operational fit for many client newsletters
3. Klaviyo — best for ecommerce clients
4. HubSpot — best for high-value enterprise retainers
5. Brevo — best margin-friendly option for smaller clients

The biggest mistake agencies make is optimizing for the cheapest per-email price instead of building clean sender reputation boundaries. The platform matters, but the account/domain architecture matters more. www.inboxally.com

If you tell me your agency model (B2B lead gen, ecommerce, newsletters, SaaS, local businesses, cold email, etc.) and approximate number of clients, I can narrow this down.

If your key requirement is each client having a genuinely separate sending reputation, I’d prioritize account/domain isolation over simply having a good email editor.

My shortlist

My pick for a traditional email-marketing agency: Campaign Monitor. Its agency setup lets you manage separate client accounts from an agency-level environment, with each client retaining its own subscribers, campaigns, settings and sending configuration. It also supports separate sending domains and client-level permissions/reporting. www.campaignmonitor.comhackceleration.com

For ecommerce clients, I'd pick Klaviyo. The important part is to give each client their own Klaviyo account and branded sending domain. Klaviyo explicitly notes that sender reputation is tied to domains/IPs, and a branded sending domain gives you control over that reputation rather than relying on a shared sending domain. help.klaviyo.com

One important distinction

If by "separate sending reputations" you mean:

Client A has a bad campaign → there is essentially zero deliverability impact on Client B

then I would not put all clients into one ESP account/list structure just because the platform supports multiple brands.

I'd structure it more like:

Agency
→ Client A account → send.clientA.com
→ Client B account → send.clientB.com
→ Client C account → send.clientC.com

Each client should ideally have:

• Separate ESP account/sub-account
• Separate sending domain/subdomain
• Separate SPF/DKIM/DMARC configuration
• Separate suppression/unsubscribe data
• Separate sending history/reputation
• Separate tracking where practical
• Separate IP infrastructure if the client's volume justifies it

That's considerably safer than relying on a platform's shared sending infrastructure. Klaviyo, for example, explicitly says that if multiple accounts use the same branded domain, the performance of all those accounts affects that domain's reputation. help.klaviyo.com

What I'd choose based on your agency model

If you're managing 5–50 SMB clients:
→ Campaign Monitor is probably the cleanest agency-oriented choice. Its dedicated client-account architecture is specifically designed around keeping client work separate while giving the agency centralized management. www.campaignmonitor.com

If you're an ecommerce/Shopify agency:
→ Klaviyo, with one account + branded sending domain per client.

If you're building complex lead nurturing/CRM automations:
→ ActiveCampaign, but I'd still maintain separate client accounts rather than trying to consolidate clients into one account. Current agency comparisons consistently identify it as stronger for automation than Campaign Monitor. www.campaignmonitor.comhackceleration.com

If you're a full-service agency selling CRM + funnels + email + SMS:
→ HighLevel becomes more interesting because the multi-client/sub-account model is central to the product. tooldecisionengine.com

One thing I'd avoid is choosing a platform solely because it says "multi-client" or "agency." The question to ask the vendor is: "If Client A generates a high complaint rate, what exact sending infrastructure and reputation signals are shared with Client B?" That's the answer that matters for deliverability.

If you tell me roughly how many clients you manage, monthly sends per client, and whether this is newsletters/opt-in marketing or cold email, I can narrow this down to the best 2–3 architectures for your agency.

If your priority is one agency managing many client accounts while keeping each client’s sending reputation isolated, I’d narrow it to GoHighLevel, HubSpot, and Campaign Monitor—with GoHighLevel probably the strongest fit for a typical marketing agency.

My ranking

1. GoHighLevel — my pick for an agency

The big advantage is its sub-account architecture. Each client can have their own account, contacts, automations, branding, etc., while you manage everything from the agency level. help.gohighlevel.com

More importantly for your question, HighLevel now supports dedicated sending domains at the sub-account level. You can give Client A their own sending domain/subdomain and Client B a completely different one. help.gohighlevel.com

It also supports assigning different dedicated domains to different types of email and distributing volume across domains. help.gohighlevel.com

Example architecture:

• Client A → email.clienta.com
• Client B → email.clientb.com
• Client C → email.clientc.com
• Each → separate HighLevel sub-account
• Each → separate authentication/DKIM/SPF configuration
• Agency → one master login

That's much closer to what you're describing than putting 20 clients into one ESP account.

One important caveat: don't use one shared sending domain for all your clients if reputation isolation is important. HighLevel explicitly notes that reputation on a shared domain is shared across sub-accounts. help.gohighlevel.com

2. Campaign Monitor — excellent if you're primarily an email agency

Campaign Monitor is worth a serious look if you're not interested in CRM/funnels/SMS and mainly want to manage email campaigns for clients.

Its agency-oriented account structure is particularly good for managing multiple brands from a master account, and recent agency-focused comparisons put it at or near the top specifically for multi-client management. hackceleration.com

I'd choose it over GHL if your agency's service is essentially:

"We manage your email marketing."

rather than:

"We manage your entire marketing automation/CRM/funnel system."

3. HubSpot — best if clients are sophisticated

HubSpot has a genuine multi-account capability where separate businesses can maintain separate HubSpot accounts while being managed through a unified organizational view. www.hubspot.com

It's powerful, but I'd consider it overkill and potentially expensive if you're managing lots of smaller clients.

What I'd avoid for this particular use case

ActiveCampaign is fantastic for automation, but I wouldn't make it the foundation of a large multi-client agency operation where account/reputation isolation is the primary requirement. Its documentation does recommend using separate subdomains to segment sending reputation, but that's different from giving every client a genuinely isolated account/infrastructure. www.hubspot.comhelp.activecampaign.com

Klaviyo is excellent for e-commerce, but I'd generally let each Shopify/e-commerce client have their own Klaviyo account rather than trying to make it your centralized agency platform.

The architecture I'd recommend

If you're building an agency that might eventually manage 10, 30, or 100+ clients, I'd prioritize this:

Agency
→ one administrative layer

Client 1
→ isolated account
→ client-owned sending domain/subdomain
→ client-specific DKIM/SPF/DMARC
→ client-specific lists
→ client-specific suppression/bounce history

Client 2
→ completely separate account/domain/reputation

…and so on.

The important distinction is that a separate "From" address isn't necessarily a separate reputation. You want the underlying sending infrastructure/domain to be properly separated.

Also, I'd generally use a dedicated sending subdomain rather than sending bulk marketing from the client's primary corporate domain. ActiveCampaign, for example, specifically recommends subdomains for separating different mail streams and reputation. www.hubspot.comhelp.activecampaign.com

Bottom line: If you're an agency and want centralized management + true client separation + separate sending reputations, I'd start with GoHighLevel. If you're an email-only agency, I'd investigate Campaign Monitor first. If you're servicing larger organizations with sophisticated CRM requirements, HubSpot becomes more compelling.

If you tell me roughly how many clients you manage (e.g. 10, 50, 200), monthly email volume, and whether they're mostly B2B or e-commerce, I can narrow this down to the best 2–3 architectures and the likely cost.

If your agency manages multiple client brands and you want each client’s sending reputation isolated, I’d prioritize platforms that support true client/sub-account separation, client-specific sending domains, permissions, and agency-level management.

My picks

🥇 Campaign Monitor — probably the best fit

For your specific requirement, Campaign Monitor is the one I'd look at first. Its agency offering is specifically designed around managing multiple client accounts: each client can have a separate account with its own subscribers, campaigns, templates, analytics and sending settings, while your agency manages them centrally. www.campaignmonitor.com

It also supports:

• Separate client accounts
• Granular permissions
• Agency-wide dashboard
• Private/white labeling
• Client-specific billing
• Unique API keys per subaccount
• Client-specific sending domains
• SPF/DKIM/DMARC authentication
• Template/section locking

The unique API key per subaccount is particularly useful if you're building an agency automation layer around it. www.campaignmonitor.com

Most importantly, don't put all your clients into one giant account/list if reputation isolation is a major concern. Campaign Monitor itself recommends authenticating each client's sending domain and using dedicated sending domains/subdomains when you need reputation separation. www.campaignmonitor.com

🥈 ActiveCampaign — better if automation is the priority

I'd choose ActiveCampaign over Campaign Monitor if your agency is doing sophisticated lifecycle automation: lead scoring, CRM workflows, behavioral branching, sales automation, etc.

It supports authenticated client sending domains with DKIM, SPF and DMARC, and ActiveCampaign explicitly recommends using subdomains to separate different mail streams and reputations. help.activecampaign.com

The catch is that its agency/multi-client structure isn't as elegant as Campaign Monitor's. You may end up managing separate ActiveCampaign accounts for clients, rather than having the same purpose-built agency hierarchy.

Also, don't confuse dedicated IP with reputation isolation. ActiveCampaign notes that domain reputation is increasingly important, and a dedicated IP is generally appropriate only for sufficiently high-volume senders. help.activecampaign.com

🥉 HubSpot — best for high-end clients

If your agency works with larger businesses where email is tied heavily to CRM, sales and customer data, HubSpot is compelling.

Its Enterprise setup can assign separate dedicated IPs to different brands, explicitly allowing different sending reputations between brands. It also supports multiple brands/business units and, where necessary, separate HubSpot accounts with multi-account management. knowledge.hubspot.com

But it's substantially more expensive and complex than I'd recommend if your main requirement is simply "agency sends email for 20 clients."

What I'd actually build

For an agency, I'd structure it approximately like this:

Agency
→ Client A account
→ Client B account
→ Client C account
→ Client D account

And for each client:

Client's own domain/subdomain
→ DKIM
→ SPF
→ DMARC
→ Client-specific tracking domain
→ Client-specific list/audience
→ Client-specific account/subaccount

That is much more important for reputation isolation than simply buying a dedicated IP. Mailbox providers increasingly evaluate domain reputation, so you want Client A's sending behavior to be associated with Client A's domain rather than your agency's domain or a common domain. help.activecampaign.com

My short answer:

• Agency-first: Campaign Monitor
• Automation-first: ActiveCampaign
• Ecommerce: Klaviyo
• Enterprise/CRM: HubSpot
• Simple SMB work: Mailchimp

If you're managing 10–100+ clients, I would lean strongly toward Campaign Monitor's agency/subaccount model rather than trying to make one normal email-marketing account serve everyone. Its current agency offering is unusually well aligned with exactly this use case. www.campaignmonitor.com

If your priority is an agency managing many clients while keeping each client's sending reputation isolated, I’d shortlist these:

My pick: HighLevel

For an agency specifically, HighLevel is probably the strongest fit.

Its architecture is built around an agency → client sub-account model. More importantly, each sub-account can have its own dedicated sending domain, which is exactly what you want for reputation isolation. HighLevel explicitly recommends not sharing sending domains across sub-accounts because one client's reputation can otherwise affect another's. help.gohighlevel.com

You can also connect your own Mailgun infrastructure, giving the agency control over sending infrastructure, domains, reputation, and volume limits. HighLevel documents using separate client domains/subdomains with Mailgun specifically to prevent one client's problems from taking down other clients. help.gohighlevel.com

A sensible architecture would be:

Agency
→ Client A sub-account → email.clientA.com
→ Client B sub-account → email.clientB.com
→ Client C sub-account → email.clientC.com

And authenticate each domain with SPF/DKIM/DMARC.

ActiveCampaign is my second choice

If the agency's core value is advanced automation rather than agency operations, I'd seriously consider ActiveCampaign.

It supports authenticated sending domains and subdomains, and ActiveCampaign specifically recommends subdomains as a way to segment different mail streams and manage reputation. help.activecampaign.com

It also has an agency/reseller Partner Portal for managing multiple accounts. help.activecampaign.com

The drawback is that it doesn't feel as naturally "multi-tenant agency" as HighLevel.

Mailchimp

Mailchimp is good if your clients primarily need:

• newsletters
• campaigns
• basic automation
• reporting
• easy client access

Its Mailchimp & Co functionality lets agencies connect and switch between client accounts without sharing credentials. mailchimp.com

However, I'd use separate client accounts, rather than putting multiple clients into audiences inside one account. Mailchimp itself notes that separate audiences are independent, but account-level separation is cleaner operationally for an agency. mailchimp.com

One important distinction

"Separate sending reputation" can mean two different things:

1. Separate domain reputation — e.g. Client A sends from email.clienta.com, Client B from email.clientb.com.

2. Separate IP reputation — each client actually sends through a different IP.

I'd prioritize separate authenticated sending domains/subdomains first. A dedicated IP isn't automatically better; ActiveCampaign, for example, cautions that a dedicated IP can actually perform worse if it isn't properly maintained. help.activecampaign.com

For a typical agency, I'd therefore do:

Client-specific domain/subdomain + DKIM + SPF + DMARC + separate platform account/sub-account.

Only move toward individual dedicated IPs when a client's volume warrants it.

Bottom line

If I were building an agency today:

1. HighLevel — best overall agency architecture
2. ActiveCampaign — best if sophisticated automation is the priority
3. Mailchimp — best for simple client campaign management
4. HubSpot — best when the agency is also managing CRM/sales operations

If you tell me roughly how many clients you manage (e.g. 10, 50, 200), monthly email volume per client, and whether these are newsletters/marketing emails or transactional emails, I can narrow this down substantially—including the best underlying sending infrastructure (SES vs Mailgun vs SendGrid vs platform-native).

If your main requirement is one agency managing many client accounts while keeping each client's sending reputation isolated, I’d shortlist these:

My pick: HighLevel for a general-purpose agency

If you're running, say, 10–50 clients, HighLevel is particularly attractive because its agency/sub-account architecture is built around exactly this model. You can give each client their own sub-account and configure a different dedicated sending domain for each one. HighLevel explicitly recommends not sharing sending domains between sub-accounts to avoid cross-account reputation problems. help.gohighlevel.comhelp.gohighlevel.com

For example:

• Client A → send.clienta.com
• Client B → send.clientb.com
• Client C → email.clientc.com

Each client therefore develops its own domain reputation rather than putting everyone behind your agency's domain.

One important distinction: separate sending domains ≠ completely separate IP reputation. With HighLevel's shared infrastructure, clients can still share sending IPs; HighLevel offers dedicated IPs when the volume justifies them. help.gohighlevel.comhelp.gohighlevel.com

If your clients are ecommerce: Klaviyo

I'd choose Klaviyo over HighLevel for an agency whose clients are primarily Shopify/DTC/ecommerce brands.

Klaviyo gives you separate client accounts and branded sending domains, plus much stronger ecommerce segmentation, flows, revenue attribution, and behavioral data. It also supports separate domains for marketing, transactional, and service traffic. help.klaviyo.com

The key caveat is that domain reputation follows the sending domain. Klaviyo notes that if multiple accounts use the same branded sending domain, the performance of those accounts can affect that domain's reputation. So for your use case, I'd give each client their own sending domain/subdomain rather than sharing one. help.klaviyo.com

If email itself is your agency's core service: Campaign Monitor

Campaign Monitor is worth a serious look if you're essentially an email marketing agency rather than a broader CRM/marketing agency. Its agency-oriented client management is specifically designed around isolated client accounts, with separate sending domains and client-level management. help.klaviyo.comhackceleration.com

What I would avoid

I wouldn't put all clients into one account/list with different segments just because it's cheaper or easier. Even if the platform technically supports it, you've created unnecessary operational and deliverability coupling.

For an agency, I'd want this architecture:

Agency master → Client account/sub-account → Client's own sending domain → Client's own audience → Client-specific authentication → Client-specific reputation

And ideally:

SPF + DKIM + DMARC + dedicated/branded sending domain + appropriate IP strategy

The domain separation is more important than simply having separate logins.

My ranking for your specific requirement

1. HighLevel — best overall agency infrastructure
2. Klaviyo — best if clients are ecommerce
3. Campaign Monitor — best if you're primarily an email/newsletter agency
4. ActiveCampaign — excellent automation, somewhat less elegant as a multi-client operating system
5. Brevo — attractive if keeping costs low is a major priority

If you tell me roughly how many clients you manage, monthly email volume per client, and whether they're mostly ecommerce/SMB/B2B, I can narrow this down to the best 1–2 options and explain the exact account/domain/IP setup I'd use to prevent reputation bleed between clients.

Yes. If your main requirement is one agency managing many clients while keeping each client’s sending reputation isolated, I’d prioritize platforms with true account/workspace separation + client-owned sending domains, rather than just “multiple lists” inside one account.

My shortlist

Campaign Monitor is probably the closest match to your specific requirement. Its agency-oriented setup provides isolated client accounts under an agency login, including separate client data/branding and sending domains. hackceleration.com

Klaviyo is my pick if your clients are primarily e-commerce brands. The important part is that each client should have their own Klaviyo account and branded sending domain, e.g. send.clientA.com versus send.clientB.com. Klaviyo explicitly says branded sending domains give you control over that domain's reputation. help.klaviyo.com

One nuance: separate sending domains are more important than separate IPs for most agencies. Klaviyo distinguishes domain reputation from IP reputation, and dedicated IPs are generally intended for higher-volume senders. help.klaviyo.com

What I'd build for an agency

For example:

• Client A → separate ESP account → send.clientA.com
• Client B → separate ESP account → send.clientB.com
• Client C → separate ESP account → send.clientC.com
• Agency has centralized administrative access
• Each client has its own lists, suppression data, authentication, domain reputation and reporting
• Don't put all clients into one ESP account and merely separate them with lists/tags

That last architecture is the one I'd avoid if reputation isolation is a hard requirement.

If you're managing 10–50+ clients, I'd lean Campaign Monitor for general email marketing, Klaviyo for Shopify/e-commerce clients, and ActiveCampaign when automation/CRM is the primary selling point. Recent agency-focused comparisons similarly put Campaign Monitor at the top for multi-client management and ActiveCampaign for automation. hackceleration.com

If you tell me roughly how many clients, monthly sends per client, and whether this is newsletters/marketing email vs. cold email, I can narrow it down to 2–3 platforms and compare their account isolation, dedicated domains/IPs, white-labeling, API, pricing, and agency/reseller model.

Yes. If your main requirement is an agency managing many clients while keeping each client's sending reputation isolated, I would prioritize account/sub-account isolation over almost everything else.

My shortlist

🥇 My pick: Campaign Monitor

For a traditional marketing agency, I'd look hardest at Campaign Monitor. Its agency setup is specifically designed around multiple client brands/accounts, allowing you to manage clients from a master login while keeping their data and sending infrastructure separated. Recent agency-focused testing also rated it highest specifically for multi-client management. hackceleration.com

The important architecture is:

Agency
→ Client A account → Client A sending domain/reputation
→ Client B account → Client B sending domain/reputation
→ Client C account → Client C sending domain/reputation

That's much safer than putting 20 clients into one ESP account and simply creating 20 lists.

🥈 Brevo is particularly interesting

If you're managing lots of clients and send volume matters, I'd seriously consider Brevo.

Brevo supports sub-accounts, and its documentation specifically describes assigning dedicated IPs to individual sub-accounts/senders. help.brevo.com

That gives you a structure like:

Agency → Sub-account per client → Client sending domain → Client IP/reputation

This is attractive if you want to keep software costs reasonable as the number of clients grows.

🛒 If your clients are ecommerce: Klaviyo

If you're primarily an ecommerce/email-retention agency, I'd choose Klaviyo instead.

The agency model is based around separate client accounts, which is exactly what you want for isolation. It also has much deeper ecommerce segmentation, flows and revenue attribution than general-purpose agency ESPs. ustechautomations.com

One important distinction: domain reputation vs IP reputation

I'd actually make separate sending domains/subdomains per client the non-negotiable requirement.

For example:

• Client A → email.clienta.com
• Client B → email.clientb.com
• Client C → email.clientc.com

Don't have:

• Client A → email.youragency.com
• Client B → email.youragency.com
• Client C → email.youragency.com

A bad sender shouldn't be able to damage everyone else's domain reputation.

And if you're sending enough volume to justify dedicated IPs, you can take the isolation further. Brevo, for example, explicitly supports assigning dedicated IPs to sub-accounts. help.brevo.com

What I would avoid

I wouldn't create one giant ESP account with a separate list for every client. Even Mailchimp's own guidance recommends separate accounts for agencies serving different businesses because of data separation and the risk that one client's behavior can affect the agency account. downloads.mailchimp.com

If you tell me roughly how many clients you manage (e.g. 5, 20, 100), monthly email volume, and whether they're mostly ecommerce or B2B, I can narrow this down to the best 2–3 options and compare their actual agency/sub-account + dedicated-domain/IP setup.