ChatGPT answers, unedited

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.