ChatGPT answers, unedited

Yes. If by “manage segmentation and split testing across many client lists” you mean an agency needs to run campaigns for multiple clients without building a custom system, there are a few good fits.

My shortlist

ActiveCampaign is probably the first one I'd investigate for a small agency. Its Partner Portal is explicitly designed for agencies/resellers managing multiple client accounts, with an account dashboard and user management. ActiveCampaign Help Center ActiveCampaign Help Center

It also has unusually flexible testing: standard campaign split tests plus automation splits of 2–10 paths, including percentage-based splits and tests that can automatically determine a winning path. ActiveCampaign Help Center ActiveCampaign

activecampaign.com

When I'd choose the others

Klaviyo makes more sense if your clients are primarily e-commerce/DTC. You can manage multiple accounts under one login, organize accounts into groups, and use sophisticated behavioral segments. Klaviyo Help Center Klaviyo Help Center Its flow A/B testing can split audiences and compare different messages, timing, discounts, or entire branches. Klaviyo Help Center

klaviyo.com

Customer.io is attractive if your agency does more sophisticated lifecycle/behavioral marketing. Its segments can be based on characteristics and behaviors and used as triggers, filters, recipient lists, and conversion criteria. Separate workspaces keep each client's people, automations, and metrics isolated. Customer.io Customer.io

customer.io

One important distinction

If you're talking about something like:

“I have 30 client lists. I want to upload/manage them centrally, create reusable audience rules, randomly split each list 50/50, run different campaigns, and see aggregate results across clients.”

—that's a slightly different problem from ordinary email marketing. You may want an agency experimentation/segmentation layer rather than just an ESP.

If you tell me what channels you're testing (email, SMS, ads, etc.), roughly how many client lists/accounts you manage, and whether each client needs their data completely isolated, I can narrow this down to 2–3 platforms and explain the workflow I'd use for each.

Yes. If by “client lists” you mean email/SMS subscriber databases, there are a few platforms that fit an agency model particularly well.

The ones I'd look at

• ActiveCampaign — probably the closest match to what you described. It has segmentation, automation, multi-path split testing, and an agency/partner portal for managing multiple client accounts. Its automation splits can distribute contacts across up to 10 paths, and its split testing can automatically select a winning version. ActiveCampaign Help Center ActiveCampaign Help Center ActiveCampaign Help Center
• Klaviyo — particularly strong if your clients are ecommerce brands. It supports multiple accounts under one login and has centralized account groups, while its flows support randomized A/B branches. Klaviyo Help Center Klaviyo Help Center Klaviyo Help Center
• Brevo — worth considering if you want something relatively straightforward. It supports segmentation and randomized segment splitting, plus conventional A/B email testing. Brevo Help Brevo Help

For a small agency, I'd structure it like this

Agency → Client accounts → Lists/contacts → Segments → Campaigns/tests

The important distinction is that I wouldn't try to keep all clients in one giant database. You generally want separate client accounts/workspaces, while giving your agency team a way to switch between them centrally. Klaviyo, for example, explicitly supports multiple accounts and account groups; ActiveCampaign has an agency/reseller portal. Klaviyo Help Center Klaviyo Help Center ActiveCampaign Help Center

If your main workflow is something like:

“I have 30 clients, each with several lists. I want to define audience segments, randomly split them 10/10/80, test different messages, automatically pick the winner, and repeat this without a lot of technical work.”

ActiveCampaign is particularly worth investigating. Its current split functionality is unusually flexible for this use case. ActiveCampaign Help Center activecampaign.com

If you tell me roughly how many clients/lists you manage, whether it's email or SMS, and whether clients need their own logins, I can narrow this down to 2–3 platforms and compare ease of use, agency management, segmentation, A/B testing, and likely cost.

Yes. If by “client lists” you mean email/SMS marketing lists, there are several platforms that fit a small agency particularly well.

The key distinction is whether you want one platform to manage many separate client accounts or whether you want to run segmentation/testing across a shared master audience.

Platforms I’d look at

Brevo is particularly interesting for a small agency because it has sub-organizations/multi-account functionality and uses an email-volume-oriented pricing model rather than simply charging for every stored contact. Its segmentation supports dynamic lists based on subscriber attributes and behavior. Brevo Sasanova

Zoho Campaigns is worth a serious look if the agency workflow itself is the priority. Its Agency Edition provides a multi-client dashboard, client permissions, shared resources, automation, segmentation and A/B testing; Zoho says agencies can initially add up to 10 clients for free. Zoho

Campaign Monitor is another particularly straightforward agency-oriented option: it explicitly supports managing separate client accounts, campaigns, automations, templates and reporting from one place. Campaign Monitor

If your main goal is split testing

There's another possibility: use a dedicated experimentation platform alongside your email platform. Convert, for example, has a centralized multi-client dashboard specifically for agencies running experiments across client accounts. A/B Testing Software

But if you're talking about email subject-line/content/segment A/B tests, I'd generally keep that functionality inside the email platform rather than adding another system.

What I'd narrow it down to

For a small agency, I'd investigate these three first:

1. Brevo — simplest combination of multi-client organization + segmentation + reasonable economics.
2. Zoho Campaigns Agency Edition — particularly compelling if you're managing lots of relatively small clients.
3. ActiveCampaign — better if you're building complex behavioral automations, lead scoring and CRM workflows rather than just campaigns. Sasanova

If you tell me roughly how many clients you manage, average list size per client, whether it's email or SMS, and what kind of split tests you want to run, I can narrow this down considerably—including the pricing model and what the agency workflow would actually look like.

Brevo is particularly interesting for a small agency because it has sub-organizations/multi-account functionality and uses an email-volume-oriented pricing model rather than simply charging for every stored contact. Its segmentation supports dynamic lists based on subscriber attributes and behavior. Brevo Sasanova

Zoho Campaigns is worth a serious look if the agency workflow itself is the priority. Its Agency Edition provides a multi-client dashboard, client permissions, shared resources, automation, segmentation and A/B testing; Zoho says agencies can initially add up to 10 clients for free. Zoho

Campaign Monitor is another particularly straightforward agency-oriented option: it explicitly supports managing separate client accounts, campaigns, automations, templates and reporting from one place. Campaign Monitor

If your main goal is split testing

There's another possibility: use a dedicated experimentation platform alongside your email platform. Convert, for example, has a centralized multi-client dashboard specifically for agencies running experiments across client accounts. A/B Testing Software

But if you're talking about email subject-line/content/segment A/B tests, I'd generally keep that functionality inside the email platform rather than adding another system.

What I'd narrow it down to

For a small agency, I'd investigate these three first:

Yes. If by “client lists” you mean email/SMS audiences, there are several good options—but the best choice depends on whether you want one centralized agency dashboard or simply a powerful platform you can replicate across clients.

My shortlist

• ActiveCampaign — probably my best overall pick for a small agency. It has strong segmentation, automation, and split testing (up to five variations), and it’s well suited to standardizing workflows across clients. ActiveCampaign BestSends
• Campaign Monitor — arguably the easiest agency-oriented option. It specifically supports separate client accounts, client permissions, reporting, billing, templates, and white-labeling from an agency setup. Campaign Monitor
• Klaviyo — excellent if most of your clients are e-commerce/Shopify brands. Segmentation and experimentation are very strong, and its newer agency/AI tooling makes cross-client management more practical. Klaviyo Help Center
• Customer.io — worth considering if your clients need sophisticated behavioral segmentation, event-triggered campaigns, and experimentation rather than traditional newsletter marketing. Customer.io
• Brevo — attractive if you're managing lots of smaller clients and keeping costs low is important. It's simpler, though, and its multi-client setup isn't as elegant as Campaign Monitor's. BestSends

What I'd choose for a small agency

If your workflow is something like:

10–30 clients → each has their own list → create segments → run A/B tests → automate campaigns → report results → repeat

I'd narrow it to:

Campaign Monitor if your priority is simplicity + managing lots of client accounts.

ActiveCampaign if your priority is segmentation + automation + testing sophistication.

Klaviyo if you're primarily an e-commerce retention agency.

There's also a separate category worth knowing about: if by “split testing” you mean website/landing-page experiments rather than email A/B tests, Convert is specifically built for agencies managing experiments across multiple client accounts. A/B Testing Software

If you tell me roughly how many clients/lists you manage, whether it's email or SMS, and whether the clients are mostly e-commerce/B2B/local businesses, I can narrow this to the 2–3 platforms that make the most sense and compare their pricing/workflow for an agency.

Yes. If you mean email/SMS marketing lists across multiple clients, there are a few good options. The key distinction is whether you want a platform that is merely good at segmentation/A/B testing, or one that is genuinely agency-friendly with multiple client accounts.

My shortlist

🥇 I'd look at ActiveCampaign first

ActiveCampaign is probably the closest match if your agency needs serious segmentation + experimentation without building a complicated tech stack.

It supports granular segmentation, automation-based branching, and currently allows up to five variations in a split test, including automatically selecting a winner. ActiveCampaign ActiveCampaign Help Center

More importantly for an agency, ActiveCampaign explicitly supports managing large numbers of client campaigns, lists, and brand identities through its agency/partner infrastructure. ActiveCampaign ActiveCampaign

activecampaign.com

I'd choose it if your workflow looks something like:

Client A → prospects / customers / VIPs → test offer A vs B → winner → automation
Client B → leads / inactive / high-value → test subject lines → nurture sequence
Client C → different segments → different journeys

🥈 Campaign Monitor if simplicity is the priority

Campaign Monitor is particularly interesting if your agency has lots of clients but doesn't need extremely elaborate automation.

Its agency offering is explicitly built around managing separate client accounts, campaigns, automations, templates and reporting from one place. Campaign Monitor

campaignmonitor.com

This is the one I'd investigate if your biggest headache is:

"I have 15–50 clients and don't want my team constantly jumping between accounts."

🥉 Brevo if cost matters

Brevo is worth considering for a smaller agency managing lots of relatively modest lists. Current agency comparisons tend to put it toward the value end of the market. The Open Rate Club Hack'celeration

One newer option worth knowing about

Audienceful is specifically advertising agency workspaces, with client separation, segmentation, A/B testing, automation and analytics. Audienceful

It could be interesting if you want something simpler than ActiveCampaign rather than another heavyweight marketing suite.

If you tell me roughly how many clients/lists you're managing (e.g. 10 clients × 5,000 contacts) and whether they're mostly e-commerce, B2B, or local businesses, I can narrow this down to the 2–3 platforms that make the most sense—and compare their agency pricing/workflow.

Yes. Assuming you mean email/SMS marketing lists, there are a few platforms that are particularly well suited to a small agency managing multiple client audiences.

My shortlist

1. ActiveCampaign — probably my first choice

ActiveCampaign is particularly interesting for an agency because it explicitly supports managing large numbers of client accounts and audiences. Its agency offering is designed around multiple client brands, lists and campaigns. ActiveCampaign ActiveCampaign

For your specific use case, the big advantage is segmentation + experimentation inside automations. You can split audiences into different paths, including percentage-based splits and up to 10 branches, which makes things like:

• 50/50 subject-line tests
• testing different offers
• testing different nurture sequences
• segmenting by engagement
• sending different creative to different customer types

fairly straightforward. ActiveCampaign Help Center

activecampaign.com

I'd choose this if your agency is doing more than basic newsletters.

2. Brevo — easiest value proposition

Brevo is worth a serious look if you're managing lots of relatively small client lists.

Its architecture supports separate client sub-organizations with their own contacts, campaigns and permissions while allowing an administrator to manage them centrally. Brevo Help

The major attraction is that Brevo's pricing is largely based around sending volume rather than simply the number of contacts, which can be much more economical when clients have big databases but don't email frequently.

brevo.com

I'd choose this if "easy and inexpensive across lots of clients" is more important than having the deepest automation system.

3. Klaviyo — excellent if your clients are Shopify/e-commerce

Klaviyo has a particularly good multi-account concept called Portfolio. It lets agencies manage multiple brands, clone campaigns/flows, share templates and segments, and view performance across accounts. Klaviyo

This is probably the strongest option if you're managing e-commerce clients where segmentation is based on things like:

• purchased product
• purchase frequency
• customer lifetime value
• browsing behavior
• abandoned carts
• VIP customers

klaviyo.com

What I'd do for a small agency

If you told me "I want one tool where my team can jump between 10–30 clients, build segments, run A/B tests, and not have to become marketing-automation experts", I'd test ActiveCampaign and Brevo first.

ActiveCampaign = better if you're selling sophisticated segmentation/automation as part of your service.

Brevo = better if you're essentially operating lots of straightforward client email programs and want to keep the economics simple.

If the clients are predominantly Shopify/e-commerce, I'd put Klaviyo at the top instead.

One important distinction: I'd look for true client/account separation, rather than putting all clients into one giant database and trying to segregate them with tags. The former makes permissions, reporting, billing, deliverability and client offboarding much cleaner.

If you tell me roughly how many clients you have, typical list size, and whether you're doing email only or email + SMS, I can narrow this down to 2–3 platforms and compare the actual agency pricing/workflow.

Yes. If by “segmentation and split testing” you mean email/SMS audiences across many client accounts, there are a few good options. For a small agency, I’d shortlist these:

• ActiveCampaign — best overall agency fit. It has an agency/partner setup specifically for managing multiple client accounts, plus fairly sophisticated segmentation, automations, and split testing. You can test up to five variations, including entire automation paths. ActiveCampaign Help Center ActiveCampaign ActiveCampaign
• Klaviyo — best if clients are ecommerce. Its Portfolio feature gives agencies a centralized view across accounts and lets you clone campaigns, flows, forms, and segments between accounts. Its A/B testing is also very polished. Klaviyo SG Klaviyo Help Center
• Customer.io — best for sophisticated behavioral segmentation. Excellent if your clients have website/app behavioral data and you want segments based on events and attributes. It supports A/B tests and multi-path/random branching in visual workflows. Customer.io Customer.io
• Campaign Monitor — worth considering for simplicity. It tends to be easier to hand off to less-technical clients, although it's less powerful than ActiveCampaign for complex automation.

What I'd choose

If you're a small generalist agency managing, say, 5–30 client lists, I'd start with ActiveCampaign. The agency account management is a major advantage: you can manage a portfolio of client accounts rather than building your own layer for keeping clients separated. ActiveCampaign Help Center ActiveCampaign

If the clients are predominantly Shopify/ecommerce brands, I'd lean Klaviyo instead. Its ability to clone segments/campaigns/flows across accounts is particularly attractive for an agency because you can develop a proven segmentation/testing framework and replicate it for each client. Klaviyo SG

If you're asking because you're considering building your own agency-facing segmentation/testing product, that's a somewhat different question—and there is an interesting gap between these tools around one reusable testing/segmentation layer across many independent client lists. I can compare the existing platforms specifically from that angle.

Yes. If by “segmentation and split testing” you mean managing email/SMS audiences for multiple client brands, there are a few good options—but they differ a lot in how well they handle the agency side.

My shortlist

• activecampaign.com — best overall fit for a small marketing agency.

It has strong segmentation, automation, and A/B testing, including up to five variations and automated winner selection. ActiveCampaign ActiveCampaign Help Center
More importantly, ActiveCampaign has an agency/partner portal for managing multiple client accounts, plus account snapshots that can clone templates, automations, lists, tags, segment conditions, etc. ActiveCampaign Help Center ActiveCampaign Help Center
I'd start here if your clients have different audiences and you want repeatable campaign/automation playbooks.

• campaignmonitor.com — easiest multi-client experience.

A recent agency-focused comparison specifically found its multi-client account management particularly strong, with separate client accounts and centralized management. Hack'celeration
I'd favor this if your agency is more campaign/newsletter focused and less interested in sophisticated behavioral automation.

• hubspot.com — best if clients need CRM + marketing.

HubSpot can organize multiple brands and, with its multi-account capabilities, manage separate organizations while providing cross-account reporting/workflows. HubSpot
But it's probably more platform than you need if the core job is simply segmentation + email experimentation.

• customer.io — best for sophisticated behavioral segmentation.

Its segmentation can use attributes and real-time behavioral events, and segments can feed campaigns, triggers, filters, and conversion goals. Customer.io Customer.io
Great technically, but I'd rank it below ActiveCampaign for a small agency where ease of onboarding many clients matters.

What I'd choose

If you're managing, say, 10–50 client lists and want your team to be able to do something like:

“Take Client A's customers who bought X but haven't bought Y in 90 days → split 50/50 → test two offers → automatically send the winner → report the result.”

I'd go with ActiveCampaign.

The particularly attractive part for an agency is that you can establish a standardized client setup and reuse automations/templates rather than reinventing everything for every account. Its current agency tooling is explicitly designed around managing client accounts. ActiveCampaign Help Center ActiveCampaign Help Center

If you tell me roughly how many clients/lists you manage, whether it's email only or email + SMS, and whether clients need their own logins, I can narrow this down to 2–3 platforms and compare pricing, multi-client management, segmentation, A/B testing, and white-label/reporting.

Yes. If by segmentation + split testing you mean email/SMS lists across multiple clients, there are several platforms that are much better suited to a small agency than trying to run everything through separate client accounts.

My shortlist

A 2026 agency-focused comparison puts ActiveCampaign at the top for automation-heavy agencies and Brevo as a particularly good value option for agencies managing many smaller accounts. Hack'celeration BestSends

Zoho Campaigns is particularly interesting for your use case. Its Agency Edition is explicitly designed for managing multiple clients from one dashboard, with separate client accounts, permissions, segmentation, automation, and the ability to allocate subscription/contact capacity across clients. Zoho

What I'd choose

If you're a small agency managing, say, 5–30 client lists, I'd narrow it to:

• Brevo → easiest/most economical if clients mostly need newsletters, segmentation, automations and straightforward A/B tests.
• ActiveCampaign → best if you're doing sophisticated behavioral segmentation and automated journeys. It's more powerful but takes longer to learn.
• Zoho Campaigns → worth a serious look if your priority is one agency console managing lots of relatively independent client accounts.
• HighLevel → best if you want to go beyond email into CRM, SMS, funnels, lead management, etc. Its agency/sub-account architecture is a major advantage. Tool Decision Engine

One important distinction: email A/B testing and website/landing-page A/B testing are very different products. If you mean testing different emails/offers against segments of each client's email list, the above are appropriate. If you mean something like "show 50% of website visitors version A and 50% version B, across 20 client websites", I'd recommend a different stack.

If you tell me roughly how many clients/lists you manage, whether they're B2B or ecommerce, and whether you need email only or email + SMS, I can narrow this down to the 2–3 platforms I'd actually trial, including likely pricing and the agency workflow.

Yes. If by “segmentation and split testing” you mean email/SMS lists across multiple clients, there are several good options. The key is choosing something with true multi-client/account management, rather than just a powerful ESP.

My shortlist

ActiveCampaign is probably my first choice if you want to build a repeatable agency process. It specifically supports managing hundreds of clients, and its segmentation/automation capabilities are substantially deeper than the simpler newsletter platforms. ActiveCampaign BestSends

Brevo is particularly attractive if you're managing many relatively small client lists. Its Agency Partner program lets an agency access and manage client accounts from the agency account. Brevo Help

Campaign Monitor is worth a serious look if ease of use and client presentation matter more than sophisticated automation. It has agency-specific account management, permissions, client billing, templates, reporting, and separate client accounts. Campaign Monitor

Klaviyo is the one I'd choose if most of your clients are ecommerce. Its Portfolio functionality lets you oversee multiple brands, clone campaigns/flows/segments between accounts, and view performance across accounts. Klaviyo UK

AWeber is an underrated option for a small agency: Team Hub lets you manage separate client accounts from one login, while still providing segmentation and fairly robust A/B testing. AWeber

But there's an important distinction

If you mean something more like:

“I have 20 clients, each with their own lists. I want one central interface where my team can build segments, launch experiments, see winners, and reuse successful tests across clients.”

Then I'd narrow it to ActiveCampaign vs. Brevo vs. Campaign Monitor.

If instead you mean:

“I want to take all my clients' audiences and run a standardized segmentation/testing program across them, potentially using different ESPs.”

That's a different category. You'd want an agency experimentation/data layer rather than simply another email platform. There are products such as EmailElement designed to manage data/brands across multiple ESPs, and experimentation platforms such as Convert designed around centralized multi-client testing. EmailElement A/B Testing Software

If you tell me roughly how many clients/lists you manage (e.g. 10, 50, 200), whether they're B2B or ecommerce, and whether you're sending email, SMS, or both, I can narrow this to the 2–3 platforms I'd actually consider and compare their pricing/workflow for an agency.

Yes. If you mean email/SMS marketing lists, there are several good options—but the best choice depends on whether you want one workspace for many clients or simply a powerful platform you can replicate across client accounts.

My shortlist

• campaignmonitor.com — best fit for a small agency

• Built specifically around agencies managing multiple client accounts.
• Separate client lists/accounts, permissions, templates, reporting, and billing from one place.
• Good segmentation and campaign testing without the complexity of an enterprise CDP.
• Particularly attractive if your team values ease of use over extremely sophisticated automation. Campaign Monitor Hack'celeration
• zoho.com — worth a serious look

• Its Agency Edition is explicitly designed for multi-client management.
• One dashboard, client separation, segmentation, automation and A/B testing.
• You can manage client access/permissions and reuse templates across clients.
• Zoho says the Agency Edition allows up to 10 clients free, then scales beyond that. Zoho
• activecampaign.com — best if automation is the priority

• Much more powerful for behavioral segmentation and complex customer journeys.
• Excellent if your agency is going beyond newsletters into lead nurturing, lifecycle marketing, CRM, etc.
• The tradeoff is that its multi-client architecture isn't as elegant as a purpose-built agency platform. BestSends
• brevo.com — best budget-oriented option

• Attractive when you have lots of relatively small client lists.
• Easier to justify economically than some contact-count-heavy platforms.
• Less sophisticated than ActiveCampaign for complex segmentation/automation. Hack'celeration
• mailjet.com — another agency-friendly option

• Uses subaccounts to keep each client's lists, data and sending reputation isolated while giving the agency centralized management. Sinch Mailjet

What I'd choose

If your requirement is essentially:

"We have 10–50 clients, each with their own email list. We want to easily segment them, run A/B tests, see results, and manage everything without becoming email-marketing-system administrators."

I'd start with Campaign Monitor and Zoho Campaigns.

If instead you're saying:

"We want sophisticated behavioral segmentation—e.g. people who opened X, bought Y, visited Z, haven't engaged for 90 days—and automated experiments/journeys."

I'd lean toward ActiveCampaign.

One important distinction: email A/B testing (subject lines, content, send time, etc.) is quite different from website/CRO split testing. If by "split testing" you mean landing pages/websites rather than email campaigns, the recommendation changes substantially; Convert is one example designed to let agencies run experiments across many client accounts from a centralized dashboard. A/B Testing Software

If you tell me roughly how many clients/lists you manage, typical list size, and whether you're doing email only or email + SMS, I can narrow this down to the 2–3 platforms I'd actually demo, including approximate costs.

Yes. If you mean email/SMS marketing lists, there are a few good options, but for a small agency managing many separate client accounts, I’d put ActiveCampaign at the top of the list.

My shortlist

• ActiveCampaign — best overall for an agency

• Built around segmentation, automations, and testing rather than just newsletters.
• Its agency/reseller Partner Portal is specifically designed to manage multiple client accounts. ActiveCampaign Help Center ActiveCampaign Help Center
• Segments can be built from fairly sophisticated conditions. ActiveCampaign Help Center
• Split testing is unusually flexible: campaigns can test subject lines/content, while automations can split into up to 10 paths and determine a winning path. ActiveCampaign Help Center ActiveCampaign Help Center
• ActiveCampaign now explicitly positions the product around managing a client portfolio from one place. ActiveCampaign
• Best if: you're running campaigns/automations for 5–50+ clients and want repeatable agency processes.
• Klaviyo — best for ecommerce clients

• Excellent segmentation and behavioral targeting; segments update dynamically based on customer behavior. Klaviyo Help Center
• Its newer account hierarchy/portfolio functionality is useful for agencies managing multiple client accounts. Klaviyo Help Center
• Strong A/B testing inside flows, including multi-branch tests. Klaviyo Help Center
• Best if: most of your clients are Shopify/ecommerce brands.
• HubSpot — best if CRM is central

• More comprehensive than you probably need if the primary job is simply list segmentation + email experimentation.
• Makes more sense if you're also managing pipelines, sales, forms, attribution, etc.
• Best if: clients expect you to manage both CRM and marketing.

What I'd choose for a small agency

If your workflow is something like:

Client A has 50k contacts → create segments → run campaigns → A/B test → report results
Client B has 15k contacts → same process
Client C has 200k contacts → same process

I'd seriously consider ActiveCampaign. The combination of separate client accounts + agency management + reusable segmentation + strong split testing is unusually well suited to that model. ActiveCampaign Help Center ActiveCampaign

If, however, these are primarily ecommerce lists, I'd lean Klaviyo.

If you tell me (1) email only vs. email+SMS, (2) roughly how many clients/lists, and (3) whether the clients are mostly ecommerce, B2B, nonprofits, etc., I can narrow it down to 2–3 platforms and compare agency pricing, ease of managing client accounts, segmentation, A/B testing, and white-label/reporting.

Yes. If you mean an agency managing segmentation, campaigns, and A/B tests for many different clients/lists, there are a few good options—but they differ quite a bit in how well they handle the agency model.

My top pick: ActiveCampaign

ActiveCampaign is probably the best fit if ease of use + agency management + sophisticated segmentation/testing are the priorities.

• Multiple client accounts: its Partner Portal is specifically designed for agencies/resellers to manage multiple client accounts from one place. ActiveCampaign Help Center ActiveCampaign Help Center
• Strong segmentation: you can build segments using contact/account data and behavioral conditions.
• Excellent split testing: campaigns can test subject lines, sender information, and content; automations can split contacts into as many as 10 paths and determine a winning path. ActiveCampaign Help Center ActiveCampaign Help Center
• Agency-friendly: ActiveCampaign explicitly positions its platform around managing a client portfolio with consistent strategies and reporting. ActiveCampaign
• Good for repeatable processes: you can build an automation/testing framework once and adapt it for each client.

activecampaign.com

Klaviyo — better if clients are ecommerce

Klaviyo is arguably better if your clients are primarily Shopify/ecommerce brands.

It has very good dynamic segmentation and allows campaigns to target multiple lists/segments while automatically deduplicating contacts. Klaviyo Help Center Klaviyo Help Center It also supports A/B testing within flows, including testing timing, content, and other flow variables. Klaviyo Help Center Klaviyo Help Center

For agencies, Klaviyo now has organizations, portfolios, and account groups, which make handling multiple client accounts considerably easier. Klaviyo Help Center Klaviyo Help Center

klaviyo.com

Customer.io — powerful, but less "simple"

Customer.io is worth considering if your clients have sophisticated behavioral data or product-led/customer lifecycle use cases.

Its segmentation is extremely flexible, with behavioral conditions and real-time customer data. Customer.io It also has separate workspaces, which can be useful for keeping clients completely isolated. Customer.io

The downside is that I'd expect a small agency to spend more time learning it than ActiveCampaign.

customer.io

How I'd choose

One important distinction: if by "many client lists" you mean you want one centralized interface where your agency can upload/manage dozens or hundreds of completely separate client lists and run standardized segmentation/A-B tests across them, that's a slightly different requirement than simply having multiple client accounts.

If that's what you're after, I can narrow this down to 3–5 platforms specifically designed for an agency/white-label multi-client workflow, including pricing and how painful it is to move between client accounts.

Yes. If by “client lists” you mean email subscriber lists/audiences, there are a few good options—but they differ a lot in how well they handle an agency’s multi-client workflow.

My shortlist

• activecampaign.com — best overall for an agency

• Strong segmentation based on tags, behavior, custom fields, engagement, etc.
• Built-in split testing, including up to 5 variations. ActiveCampaign ActiveCampaign
• Specifically has an agency/reseller program designed around managing many client accounts, with centralized account controls and white-label capabilities. ActiveCampaign
• Better choice if you want sophisticated automation in addition to newsletters.
• mailchimp.com — easiest to learn

• Very approachable interface for smaller teams.
• Good segmentation and straightforward A/B testing of subject lines, content, sender names, send times, etc. Mailchimp
• I'd choose this if your agency mostly does campaign → segment → test → report, rather than complicated behavioral automation.
• The downside is that multi-client administration can become cumbersome as the number of clients grows.
• hubspot.com — best if clients need a broader CRM

• Particularly strong if you're managing email alongside CRM, sales pipelines, forms, landing pages, etc.
• Its current multi-account management can connect up to 30 separate HubSpot accounts from a centralized configuration space, with shared assets/workflows and portfolio-level visibility. HubSpot HubSpot Knowledge Base
• Probably overkill if all you need is email segmentation/testing.

One important distinction

If you're asking for one agency dashboard where you can manage 20–50 different clients/lists without constantly logging in and out, I'd lean toward ActiveCampaign rather than simply picking the easiest email platform. ActiveCampaign explicitly positions its agency offering around managing multiple client accounts and campaigns. ActiveCampaign

If you're asking for something more like:

“I upload/import each client's list, create reusable segments, run A/B tests, and have my team manage everything from one simple interface.”

then there are some newer agency-specific platforms worth looking at too. For example, Spreeflo is explicitly built around multi-client email workflows, segmentation, templates, automations, and A/B testing. Spreeflo

For a small agency, my ranking would be:

1. ActiveCampaign — best balance of agency management + segmentation + testing
2. Mailchimp — easiest for a less technical team
3. HubSpot — best if email is part of a larger CRM operation
4. Agency-specific newer tools — potentially much simpler/cheaper, but I'd evaluate them carefully for deliverability and maturity

If you tell me roughly how many clients you have (e.g. 10, 50, 200), average list size, and whether you're doing email only or SMS too, I can narrow this down to the 2–3 platforms that make the most sense and compare their multi-client pricing/workflow.

Yes. For a small agency managing multiple client lists, segmentation, and split tests, you’ll usually want something with:

• separate client workspaces/accounts
• audience segmentation (tags, behaviors, custom fields)
• A/B testing for campaigns (email/SMS/web)
• client permissions/reporting
• templates and repeatable workflows

The best choice depends on whether you mean email/list marketing split tests or website/CRO split tests.

If you mean email lists + campaigns

ActiveCampaign

activecampaign.com

Good fit for agencies that run lifecycle marketing:

• strong segmentation
• automations and customer journeys
• A/B testing for emails
• CRM + marketing together
• agency/partner capabilities for managing many clients ActiveCampaign

Best for: agencies doing retention, nurture, lead funnels, ecommerce, B2B.

Zoho Campaigns

zoho.com

A more budget-friendly option:

• agency dashboard
• multiple client accounts
• segmentation
• automation
• A/B testing workflows Zoho

Best for: small agencies that need simplicity and reasonable costs.

Klaviyo

klaviyo.com

Excellent if your clients are ecommerce brands:

• very powerful segmentation
• predictive analytics
• email/SMS experiments
• Shopify ecosystem

Downside: can become expensive as lists grow.

If you mean website landing page / conversion split testing

Convert.com

convert.com

Probably one of the more agency-oriented CRO tools:

• multiple client projects
• centralized management
• permissions
• experiment reporting A/B Testing Software

Best for: agencies selling CRO retainers.

Varify.io

varify.io

Designed around agency workflows:

• multi-project management
• visual editor
• client separation
• predictable project pricing Varify.io - A/B Testing Platform

My shortlist for a small agency

If you’re a small agency managing 5–50 client databases, I’d generally avoid enterprise tools like Optimizely unless a client specifically requires them. They tend to add cost and operational overhead.

A useful question: are your “client lists” mostly email/SMS audiences, or are you trying to run landing-page/website A/B tests? The ideal stack is different.

Yes. If by “client lists” you mean email/contact databases and you want segmentation + A/B testing + automations in one place, I’d look first at ActiveCampaign.

Best fit: ActiveCampaign

activecampaign.com is unusually well suited to an agency model:

• Segmentation: granular segments based on contact attributes, behavior, engagement, etc. ActiveCampaign
• A/B testing: test up to 5 variants of campaigns, including subject lines, content, CTAs, and more. ActiveCampaign ActiveCampaign Help Center
• Automation splits: you can split contacts into as many as 10 paths, including percentage-based splits and tests that automatically select a winning path. ActiveCampaign Help Center ActiveCampaign Help Center
• Agency-oriented: ActiveCampaign specifically positions its platform around managing hundreds/thousands of clients, campaigns, lists, and brands. ActiveCampaign ActiveCampaign
• Reusable workflows: this is particularly useful if your agency runs similar campaigns for many clients.

Two alternatives worth considering

Customer.io is stronger if your clients have sophisticated behavioral/product data. Its segmentation is extremely flexible, and its workflow builder supports A/B tests, multi-way branches, and behavioral triggers. Customer.io Customer.io It also has separate workspaces, which can be useful for keeping client data isolated. Customer.io

Varify.io is worth looking at if by “split testing” you mean website/CRO A/B testing, rather than email testing. It's specifically designed around agencies managing multiple client projects. Varify.io - A/B Testing Platform

My quick recommendation

If you tell me roughly how many clients/lists you manage, whether these are email lists, SMS lists, or both, and whether clients need separate logins, I can narrow this down to 2–3 platforms and compare pricing, client isolation, white-labeling, reusable campaigns, segmentation, and A/B testing.

Yes. If by “segmentation and split testing” you mean managing multiple clients’ email/contact lists, creating audience segments, and running A/B tests without a lot of technical overhead, there are a few good options.

My shortlist

What I'd look at first

1. Campaign Monitor — probably the closest fit if email is your core service.
It has an agency-oriented model where you can manage separate client accounts, brands, templates and reporting. A recent 2026 comparison specifically found it strongest for multi-client agency management. Hack'celeration

2. Brevo — best value if you're managing lots of lists.
Brevo is particularly interesting because it doesn't force you to pay simply for storing a large number of contacts. It also supports segmentation and lets you create split lists for controlled testing—for example, testing two versions on 10% of a segment before sending the winner to the remainder. Brevo Help EmailTooltester.com

3. ActiveCampaign — best if your segmentation gets sophisticated.
If you're doing things like:

Client A → prospects → opened last 3 emails → didn't click → attended webinar → send Version B

…ActiveCampaign is excellent. Its combination of tags, custom fields, behavioral triggers, automations and A/B testing is substantially more powerful than basic newsletter platforms. The downside is that it can become more complicated operationally when you're managing lots of separate client accounts. EmailTooltester.com InVader Marketing

4. HighLevel — worth considering if you want an actual agency operating system.
If you're not just doing email—e.g. email + SMS + CRM + forms + funnels + client reporting—HighLevel is probably more compelling. Its major advantage is that the platform is designed around multiple client sub-accounts rather than treating the agency as an afterthought. Tool Decision Engine

The key question

There's an important distinction between:

A. “I need to manage many clients' email accounts.”
→ Campaign Monitor / Brevo / ActiveCampaign

and

B. “I want one agency platform where I can create a client, import their list, segment it, run campaigns/tests, and manage everything from one master dashboard.”
→ HighLevel becomes much more interesting.

If you tell me roughly how many clients/lists you manage (e.g. 10, 30, 100), average list size, and whether these are email-only or email + SMS, I can narrow it down to 2–3 platforms and compare the actual workflow and pricing.

Yes. For a small agency managing multiple client lists, segments, and A/B tests, you’ll usually want something that supports:

• separate client workspaces/accounts
• audience segmentation
• randomized splits/experiments
• reusable campaign templates
• client-safe reporting
• permissions so clients only see their own data

The best fit depends on whether you mean email/customer lists or website/app experimentation:

If you mean email lists + marketing campaigns

HubSpot

Good all-around agency choice if clients need CRM + segmentation + email testing. You can manage multiple client portals, build lists based on properties/behavior, and run email A/B tests.

Best for:

• lead nurture
• lifecycle marketing
• CRM-driven segmentation
• agencies already offering marketing automation

Downside: can become expensive as client count grows.

Klaviyo

Strong for ecommerce clients. Segments are powerful (purchase history, engagement, predicted value), and it has built-in campaign and flow testing.

Best for:

• Shopify/DTC brands
• revenue-focused email programs

Customer.io

A flexible option for agencies handling more technical clients. It supports behavioral segmentation and message experiments.

Best for:

• SaaS
• apps
• complex user journeys

If you mean website conversion testing + visitor segmentation

Convert Experiences

Designed with agencies in mind: multiple client projects, centralized management, permissions, and experiment organization. A/B Testing Software

Zoho PageSense

A simpler, more approachable option for agencies that need A/B tests, funnels, heatmaps, and client management without enterprise complexity. Zoho

Varify.io

Another agency-oriented testing platform with multi-project management, visual editing, and client reporting workflows. Varify.io - A/B Testing Platform

If you want an agency "command center"

For a small agency, I’d shortlist:

A practical agency setup is often:

• CRM/email platform → owns segments and customer lists
• A/B testing platform → owns experiments
• dashboard/reporting layer → gives clients a clean view

If you tell me whether your “client lists” are email/SMS audiences or website visitors, and roughly whether you manage 5 clients vs. 50+, I can narrow this to 2–3 options.

Zoho PageSense

A simpler, more approachable option for agencies that need A/B tests, funnels, heatmaps, and client management without enterprise complexity. Zoho

Varify.io

Another agency-oriented testing platform with multi-project management, visual editing, and client reporting workflows. varify.io

If you want an agency "command center"

For a small agency, I’d shortlist:

A practical agency setup is often:

Yes. If by “many client lists” you mean an agency managing separate email audiences for multiple clients, there are a few good options—but ActiveCampaign is probably the closest fit.

My shortlist

• activecampaign.com — best overall for an agency

• Strong segmentation, tagging, behavioral triggers, automations, and split testing.
• Its agency program is specifically designed for managing multiple client accounts, including centralized client management and reselling accounts. ActiveCampaign ActiveCampaign Help Center
• It currently supports testing up to five variations, not just traditional A/B tests. ActiveCampaign ActiveCampaign
• You can also create a “sandbox” account with reusable campaigns, templates, automations, lists, tags, segment conditions, etc., which is useful if you repeatedly onboard similar clients. ActiveCampaign Help Center
• Best if: you're building repeatable processes across, say, 10–100+ clients.
• mailchimp.com — easiest to learn

• Probably the gentlest interface for a small agency and clients who want to understand what you're doing.
• Mailchimp & Co lets agencies connect and switch between client accounts without sharing passwords. Mailchimp
• Its A/B testing can test subject lines, content, images, send times, etc., against audience segments, with automatic winner selection. Mailchimp
• Best if: simplicity and client familiarity matter more than sophisticated automation.
• Aledamail — interesting if you want an actual agency “master platform”

• It is built around isolated client workspaces, one agency login, white-labeling, per-client usage, and a single agency bill. Aledamail
• Best if: you want clients to experience your agency's platform rather than having you manage a collection of conventional ESP accounts.
• I'd investigate its deliverability/reputation infrastructure carefully before moving a large portfolio onto it.

What I'd choose

For a small agency managing multiple clients, I'd probably start with ActiveCampaign.

The important distinction is that you're not really looking for “an email platform with segmentation.” You're looking for an agency operating system for email:

Client → audience → segments → campaign → A/B test → winner → reporting

ActiveCampaign is unusually well aligned with that workflow, and its agency program offers discounts as you add accounts—25% for 2–24 accounts, scaling up to 55% for 100+ accounts under the documented agency pricing. ActiveCampaign Help Center

If you tell me roughly how many clients/lists you manage (e.g. 5, 20, 100), whether they're B2B or ecommerce, and whether you need white-labeling, I can narrow this down to 2–3 platforms and compare pricing + segmentation + A/B testing + multi-client management.

Yes. For a small agency managing multiple client lists, audience segments, and A/B tests, you’ll usually want a platform with:

• multi-client workspaces/accounts
• segmentation by tags, behaviour, purchase history, engagement, etc.
• A/B testing
• client permissions/reporting
• templates and automations you can reuse

The best fit depends on whether you mean email marketing lists or website/app conversion testing.

Best all-round options for an agency

Klaviyo

Best for: e-commerce agencies and clients with customer data.

Pros

• Excellent segmentation (events, purchases, behaviour)
• Strong automated flows
• Good A/B testing
• Client accounts can be separated cleanly

Cons

• Can get expensive as contact databases grow
• More powerful than many small agencies need

Good if your clients are Shopify/DTC brands.

ActiveCampaign

Best for: small agencies managing lead nurture and CRM-style campaigns.

Pros

• Very strong automation builder
• Easy tagging and segmentation
• Good balance of power and usability
• Suitable for many industries

Cons

• Agency account management is less elegant than some agency-first tools

Zoho Campaigns

Best for: budget-conscious agencies with many smaller clients.

It has an agency-focused setup with multi-client management, segmentation, automation, and A/B testing workflows. Zoho

Pros

• Designed around managing multiple clients
• Lower cost than many enterprise tools
• Simple interface

Cons

• Less sophisticated segmentation than Klaviyo

HubSpot

Best for: agencies that also manage CRM, sales pipelines, and reporting.

Pros

• Excellent client visibility
• Great reporting
• Powerful lists and workflows

Cons

• Pricing rises quickly
• Can be overkill for simple email campaigns

If you mean website/app split testing (CRO)

For testing landing pages, offers, layouts, and user journeys:

Convert.com

Good agency choice because it supports multi-client management, separate projects, permissions, and centralized control. A/B Testing Software

Varify.io

More lightweight and agency-oriented, with multi-project management and visual testing tools. Varify.io - A/B Testing Platform

My shortlist for a small agency

A common agency stack is:

• ActiveCampaign/Klaviyo → segmentation + email experiments
• GA4 + Convert/Varify → website split testing
• Looker Studio → client reporting

If you tell me roughly how many clients/lists you manage (e.g., 5 vs 50), whether they are ecommerce or lead-gen, and your monthly budget, I can narrow it to 2–3 realistic choices.

Good agency choice because it supports multi-client management, separate projects, permissions, and centralized control. convert.com

Varify.io

More lightweight and agency-oriented, with multi-project management and visual testing tools. varify.io

My shortlist for a small agency

A common agency stack is:

Yes. If you mean email/CRM lists, there are a few platforms that fit a small agency particularly well. The key distinction is whether you want one agency dashboard managing many clients versus simply having good segmentation/testing inside each client account.

My shortlist

1. ActiveCampaign — probably the closest match

activecampaign.com

I'd start here if you're managing lots of relatively small client lists.

It has unusually strong agency-oriented tooling: ActiveCampaign explicitly supports managing large numbers of client campaigns/accounts, and its agency sandbox/snapshot functionality lets you create reusable templates containing campaigns, automations, lists, tags, segment conditions, fields, etc. ActiveCampaign ActiveCampaign Help Center

For testing, you can do up to five variations, including testing content, subject lines, CTAs, send times, and entire automation paths. ActiveCampaign ActiveCampaign Help Center

The really useful agency workflow would be:

Client → audience/segment → campaign → randomized split → winner → automated follow-up

And you can reuse the underlying structure for the next client.

2. Klaviyo — easiest if these are ecommerce clients

klaviyo.com

If your agency mostly works with Shopify/ecommerce brands, I'd seriously consider Klaviyo instead.

Its Portfolio feature is specifically designed for managing multiple brand accounts from a centralized view. You can monitor multiple accounts, manage users, and clone campaigns, flows, templates, and segments across accounts. Klaviyo Klaviyo Academy

Its flow builder also supports randomized splits for testing different journeys, timing, discounts, additional emails, etc. Klaviyo Help Center

This is probably the nicest UX of the options, but it's more opinionated around ecommerce/customer data.

3. Customer.io — best if segmentation is the core product

customer.io

Customer.io is excellent if by "segmentation" you mean things like:

• visited pricing page but didn't convert
• signed up 7–14 days ago
• used feature X but not feature Y
• high-value customers who haven't purchased recently
• behavioral/event-based cohorts

Its segmentation engine supports attributes, page views, behavioral events and essentially arbitrary first-party data. Customer.io

It also has A/B testing, random cohorts, multi-way branches and visual workflows. Customer.io

The catch for an agency is that its workspaces are deliberately isolated: each workspace has its own profiles, automations and metrics. That's good for client separation, but less convenient if your main goal is a single centralized agency operating layer. Customer.io

What I'd choose

If I were setting this up for a small agency with, say, 10–50 client lists, I'd rank them:

1. ActiveCampaign → best general-purpose agency platform
2. Klaviyo → best if clients are primarily ecommerce
3. Customer.io → best if sophisticated behavioral segmentation matters more than simplicity
4. HubSpot → best if you also need a full CRM/sales platform

One thing I'd specifically investigate before choosing is whether you want the agency to own/manage the client accounts, or whether each client should own their account and merely give your agency access. That changes the ideal architecture quite a bit.

If you tell me roughly how many clients/lists you manage, list size, and whether they're ecommerce/B2B/newsletters/leads, I can narrow this down to 1–2 platforms and suggest the simplest agency setup.

2. Klaviyo — easiest if these are ecommerce clients

klaviyo.com

If your agency mostly works with Shopify/ecommerce brands, I'd seriously consider Klaviyo instead.

Its Portfolio feature is specifically designed for managing multiple brand accounts from a centralized view. You can monitor multiple accounts, manage users, and clone campaigns, flows, templates, and segments across accounts. klaviyo.com academy.klaviyo.com

Its flow builder also supports randomized splits for testing different journeys, timing, discounts, additional emails, etc. help.klaviyo.com

This is probably the nicest UX of the options, but it's more opinionated around ecommerce/customer data.

3. Customer.io — best if segmentation is the core product

Yes. If by “segmentation and split testing” you mean email lists/audiences, there are several platforms that are much easier to operate across many clients than giving every client a completely separate login.

My shortlist

• campaignmonitor.com — probably the best fit if your priority is simple client management. It supports separate client accounts/lists, permissions, automation, reporting, and agency/white-label features. Campaign Monitor
• zoho.com — particularly interesting for a small agency on a budget. Its Agency Edition gives you a multi-client console, centralized templates, segmentation, automation and A/B testing. It currently allows up to 10 client accounts on the free agency allocation. Zoho
• activecampaign.com — my choice if segmentation and sophisticated automation are more important than having the slickest agency dashboard. It gets considerably more powerful when you're building behavioral segments, conditional journeys, lead scoring, etc. Hack'celeration Tool Decision Engine
• brevo.com — worth considering if you have lots of relatively small client lists and want to keep costs down. Agency-oriented comparisons currently rate it highly for boutique agencies. Hack'celeration
• transpond.io — an interesting newer option if you want one agency workspace with client-specific campaigns, automations, reporting and white-labeling. Transpond

The distinction I'd make

If your agency looks like:

15 clients → each has a 5k–50k subscriber list → you build campaigns → segment audiences → run subject-line/content A/B tests → report results to each client

I'd lean toward Campaign Monitor or Zoho Campaigns for simplicity.

If it looks more like:

“People who bought X but haven't bought Y in 90 days, except customers acquired through campaign Z, enter this workflow; split them based on behavior; then test different offers…”

I'd lean toward ActiveCampaign.

And if you're talking about website landing-page/conversion A/B testing, rather than email A/B testing, that's a different category of tools entirely.

If you tell me roughly how many clients, average list size, and whether these are email newsletters vs. ecommerce/CRM lists, I can narrow it down to the 2–3 platforms I'd actually consider for your agency, including approximate cost.

Yes. If you mean email/SMS lists and you want a small agency to manage segmentation + A/B testing across many clients without constantly logging into separate systems, there are a few good options.

My shortlist

🥇 I'd look at Zoho Campaigns first

zoho.com is unusually well aligned with what you're describing.

It gives an agency one dashboard for multiple client accounts, with separate client data, permissions, campaigns and reporting. It also supports segmentation, automation and A/B testing. Their Agency Edition currently allows up to 10 clients free, with the ability to scale beyond that. Zoho

The big advantage is that you don't have to construct an elaborate marketing-ops architecture just to manage clients.

🥈 Campaign Monitor

campaignmonitor.com is probably the one I'd consider if client experience and simplicity matter more than extremely sophisticated segmentation.

Each client can have their own lists, campaigns, templates, analytics and sending settings, while your agency manages them centrally. It also has client-specific billing, permissions and white-label capabilities. Campaign Monitor

🥉 Customer.io

customer.io is the more powerful choice if by "segmentation" you mean things like:

People who bought X, haven't bought Y, visited the site 3 times, opened the last two emails, but haven't clicked in 30 days.

Its segmentation is event/behavior based, and it has built-in A/B testing and conversion tracking. Customer.io

The downside is that it's more of a marketing automation platform than a dead-simple agency email tool.

One important distinction

If you're talking about website/CRO split testing rather than email split testing, I'd look at Convert instead. It is explicitly designed for agencies running experiments across many client accounts, with centralized client management, isolated projects, permissions and reusable experiment templates. A/B Testing Software

convert.com

If I were choosing for a small agency:

• 10–30 email clients: Zoho Campaigns
• Clients care a lot about polished reporting/white-labeling: Campaign Monitor
• Advanced behavioral segmentation + lifecycle automation: Customer.io
• Email + CRM + broader marketing services: HubSpot
• Website/CRO experimentation: Convert

If you tell me what kind of lists you manage (e.g. cold leads, newsletters, ecommerce customers, B2B leads) and roughly how many client accounts, I can narrow this down to 2–3 platforms and compare their pricing/workflow.

🥇 I'd look at Zoho Campaigns first

zoho.com is unusually well aligned with what you're describing.

It gives an agency one dashboard for multiple client accounts, with separate client data, permissions, campaigns and reporting. It also supports segmentation, automation and A/B testing. Their Agency Edition currently allows up to 10 clients free, with the ability to scale beyond that. Zoho

The big advantage is that you don't have to construct an elaborate marketing-ops architecture just to manage clients.

🥈 Campaign Monitor

campaignmonitor.com is probably the one I'd consider if client experience and simplicity matter more than extremely sophisticated segmentation.

Each client can have their own lists, campaigns, templates, analytics and sending settings, while your agency manages them centrally. It also has client-specific billing, permissions and white-label capabilities. Campaign Monitor

🥉 Customer.io

customer.io is the more powerful choice if by "segmentation" you mean things like:

People who bought X, haven't bought Y, visited the site 3 times, opened the last two emails, but haven't clicked in 30 days.

Its segmentation is event/behavior based, and it has built-in A/B testing and conversion tracking. customer.io

The downside is that it's more of a marketing automation platform than a dead-simple agency email tool.

One important distinction

If you're talking about website/CRO split testing rather than email split testing, I'd look at Convert instead. It is explicitly designed for agencies running experiments across many client accounts, with centralized client management, isolated projects, permissions and reusable experiment templates.

Yes. If by “segmentation and split testing” you mean email/SMS lists for multiple clients, there are a few good options—but the best one depends on whether you want one agency dashboard or simply a powerful platform you can replicate across clients.

My shortlist

• activecampaign.com — best overall for a small agency.

Excellent segmentation, behavioral automation, and A/B testing. It now supports up to five-way split testing and multi-path automation splits. ActiveCampaign ActiveCampaign
The downside: client accounts aren't as elegantly consolidated as an agency-native platform, so managing lots of separate clients can become cumbersome. US Tech Automations

• campaignmonitor.com — best if simplicity and client management are priorities.

It has particularly strong agency-oriented multi-client account management, with clients kept separate while being manageable from an agency account. The Open Rate Club Hack'celeration
I'd favor this if your clients mostly need newsletters, segmentation, and straightforward A/B tests rather than elaborate behavioral journeys.

• gohighlevel.com — best if you want an agency operating system.

The sub-account model is designed around agencies: separate client environments, permissions, CRM, email, SMS, funnels, and automation. Tool Decision Engine
It's more platform than you need if you're just doing email/list management, though.

• customer.io — best for sophisticated segmentation.

Very good if segmentation is based on events/behavior rather than simple tags and fields. It supports multi-split journeys, A/B testing, and dynamic audience segmentation. It also added easier switching between multiple client accounts in 2026. Customer.io Customer.io Customer.io

What I'd choose

If you're a small agency managing, say, 5–30 client lists, I'd probably start with ActiveCampaign unless your clients are primarily ecommerce.

The important distinction is:

Do you want to manage many clients from one agency workspace, or do you want one powerful email engine that you duplicate for each client?

If you tell me roughly how many clients/lists you have, average list size, whether it's email/SMS/both, and whether you're doing ecommerce or lead-gen, I can narrow this down to 2–3 platforms and compare ease of use + client management + segmentation + A/B testing + cost.

Yes. If you mean email/SMS lists across multiple agency clients, there are a few good options, but the best choice depends on whether you care more about easy multi-client management or sophisticated segmentation/testing.

My shortlist

Campaign Monitor is probably the closest match to "easy to use across many client lists." It has separate client accounts, centralized agency management, permissions, client-specific billing, templates and reporting. www.campaignmonitor.com

ActiveCampaign is my pick if segmentation and experimentation are the core requirement. Its split testing can test up to five variations and can extend beyond subject lines/content into automation paths and entire journeys. www.activecampaign.com

Brevo is worth looking at if you're managing lots of relatively small clients and want to keep costs down. Its Agency Partner setup lets you switch between client accounts from the agency account and give your team access to those accounts. help.brevo.com

One important distinction

If by "split testing" you mean:

Take each client's list → automatically create segments → run experiments → determine the winner → roll the winner out → report results to the client

then I'd lean ActiveCampaign.

If you mean:

I have 20+ clients and just want a really clean way to keep their lists, campaigns, users, billing and reporting separate

I'd lean Campaign Monitor.

If you tell me roughly how many clients/lists you manage, typical list size, and whether these are ecommerce or B2B/service clients, I can narrow it down to 2–3 platforms and compare pricing + segmentation + A/B testing + multi-client workflow specifically for your agency.

Yes. If by “many client lists” you mean an agency managing multiple clients, each with their own email/SMS lists, there are a few good options—but I’d narrow it to ActiveCampaign and Klaviyo.

My top picks

I'd probably start with ActiveCampaign for a general agency. Its agency program is specifically designed around managing/reselling multiple client accounts, with a central account dashboard. It also supports split testing of up to five variations and testing entire automation journeys. help.activecampaign.com

ActiveCampaign

Klaviyo is arguably better if your clients are ecommerce brands. Its Portfolio feature gives an agency a centralized view of multiple client accounts, and you can clone segments, campaigns, flows, forms, and templates across accounts—up to 100 accounts at once. help.klaviyo.com It also has robust A/B testing for campaigns and flow branches. help.klaviyo.com

Klaviyo

One important distinction

If you're imagining something like:

Client A → upload list → create segments → split 50/50 → send A/B campaign → automatically pick winner
Client B → same workflow
Client C → same workflow

then ActiveCampaign is probably the easiest starting point.

If instead you're imagining:

Import customer data → build sophisticated behavioral segments → run recurring experiments across email/SMS → analyze revenue per segment

I'd lean Klaviyo, particularly for ecommerce.

Klaviyo's architecture keeps each client's profiles, lists, segments, campaigns, flows, integrations, and billing in separate accounts, while its Portfolio layer lets an agency manage those accounts collectively. help.klaviyo.com

If you tell me roughly how many clients/lists you manage (e.g. 10 clients × 5 lists each) and whether this is email only, SMS, or both, I can recommend the simplest setup and compare the likely costs.

Yes. If you mean email/SMS marketing lists, there are a few good options, and the best one depends on whether you want one dashboard for many clients or separate client accounts that you can manage centrally.

My shortlist

My first choice: ActiveCampaign

For the specific workflow you described, I'd probably start with ActiveCampaign.

It has strong segmentation, automations, and split testing—including testing subject lines, content, CTAs, send times, and entire automation paths. It can test up to five variations and automatically select the winner based on your chosen metric. ActiveCampaign ActiveCampaign Help Center

More importantly for an agency, ActiveCampaign has an Agency/Partner setup specifically for managing multiple client accounts. Its current Partner Portal gives agencies a searchable/sortable view of their client accounts, while the agency program supports discounted pricing as the number of resold accounts grows. ActiveCampaign Help Center ActiveCampaign Help Center

It also has account snapshots/sandboxes, so you can create a standard setup containing things like lists, automations, templates, tags, custom fields, and segment conditions, then use that as the starting point for new clients. ActiveCampaign Help Center

activecampaign.com

If simplicity is more important: Campaign Monitor

I'd look seriously at Campaign Monitor if your agency mostly does:

import list → segment → create campaign → A/B test → report → repeat

rather than complicated customer journeys.

An agency-focused 2026 comparison found Campaign Monitor particularly strong for multi-client management, with separate client environments and centralized agency administration. Hack'celeration

If you want an "agency operating system": HighLevel

HighLevel is worth considering if email is only one piece of what you do. It combines client accounts, CRM, funnels, email/SMS, automation, etc. Several current agency comparisons put it at the top when the goal is to run lots of client marketing operations from one system. Tool Decision Engine

gohighlevel.com

One important distinction

If by "many client lists" you mean something like:

20 clients × 5–10 lists each × recurring segmentation and A/B testing

I'd favor a platform with true account/workspace separation, rather than putting all clients into one giant database and trying to keep them separated with tags.

ActiveCampaign's agency architecture is particularly suited to that model.

If you tell me roughly how many clients/lists you manage, whether they're B2B or ecommerce, and whether you need email only or email + SMS, I can narrow this down to the 2–3 platforms I'd actually consider and compare their likely costs/workflow for your agency.

Yes. If you mean an agency managing segmentation + A/B testing for many separate client email lists, there are a few good options, and I’d narrow it to these:

My first choice: ActiveCampaign

For what you're describing, ActiveCampaign is probably the sweet spot.

It explicitly supports agencies managing hundreds of clients, while giving you sophisticated segmentation and automation. www.activecampaign.comwww.audienceful.com Its split testing can test up to five variations, including email content, subject lines, CTAs, send times, and entire automation paths. www.activecampaign.com

It also recently added percentage-based and multi-path splits, allowing up to 10 paths in an automation. help.activecampaign.com

ActiveCampaign

If agency management is the #1 concern: HighLevel

HighLevel is particularly interesting if you want each client essentially isolated into its own account/workspace while your agency manages everything centrally.

Its segments are dynamic and can be based on tags, fields, activity, location, engagement, etc., and they update automatically. help.gohighlevel.com It also has built-in email A/B testing, including up to six subject-line variations in its newer Campaign Agent workflow. help.gohighlevel.com

HighLevel

The sleeper option: Audienceful

If by "easy-to-use" you really mean "I don't want my team spending weeks learning the platform," I'd look closely at Audienceful.

It specifically offers agency workspaces, client list imports, segmentation, automation, A/B testing and analytics. www.audienceful.com It's much more focused on email/list management than being a giant CRM.

Audienceful

One important distinction

I'd choose differently depending on your agency model:

• 10–100+ clients, email marketing is the core service → ActiveCampaign
• You manage CRM + email + SMS + funnels + lead nurturing → HighLevel
• Small team, primarily newsletters/email campaigns, want simplicity → Audienceful
• Mostly Shopify/ecommerce brands → Klaviyo
• Clients expect sophisticated CRM/reporting/enterprise integrations → HubSpot

If you tell me roughly how many clients/lists you manage, typical list sizes, and whether you're sending email only or also SMS, I can narrow this down to the 2–3 platforms that make the most sense economically and operationally.

Yes. If you mean email/SMS lists across multiple clients, there are a few good options—but the best one depends on whether you care more about agency management or sophisticated segmentation/testing.

My shortlist

A 2026 agency-focused comparison specifically puts Campaign Monitor at the top for multi-client management, ActiveCampaign for automation, and Brevo for value. hackceleration.com

If I were running a small agency

I'd probably start with Brevo if your requirements are:

• 5–30 client lists
• separate client audiences
• easy segmentation/tagging
• straightforward A/B tests
• relatively junior staff operating the system
• keeping software costs under control

Brevo supports advanced segmentation, unlimited segments, automation, and A/B testing, and its higher-end offering supports multiple brands/business units under one master account. www.brevo.com

If segmentation + sophisticated split testing is the heart of your service, I'd lean toward ActiveCampaign. It supports considerably more sophisticated automation testing—including multiple variants within workflows—and has very strong behavioral segmentation. www.activecampaign.com

If your clients are mostly Shopify/ecommerce, I'd choose Klaviyo instead. Its segmentation and behavioral data are particularly strong for ecommerce, and agency setups can manage multiple client accounts while keeping their data/billing separated. ustechautomations.com

One important distinction

If by "split testing" you mean testing email campaigns—subject line A vs. B, creative A vs. B, different segments, different automation paths—I'd recommend the platforms above.

If you mean website/landing-page A/B testing across many clients, that's a different category. Something like Varify, VWO, or Convert is more appropriate; agency-oriented platforms are specifically designed around separate client projects and reporting. www.brevo.comwww.activecampaign.comvarify.io

If you tell me roughly how many clients/lists you manage, average list size, whether they're B2B or ecommerce, and whether you're doing email only or email + SMS, I can narrow this down to the 2–3 platforms I'd actually consider and compare their pricing/workflow.

Yes. If you mean email/SMS marketing lists, there are a few platforms that fit an agency much better than trying to manage everything inside one giant list.

My shortlist

ActiveCampaign is probably the first one I'd investigate. It explicitly supports agencies managing hundreds of clients, contacts, campaigns and brand identities, while giving you sophisticated segmentation and automation. www.activecampaign.com www.sequenzy.com

HighLevel is particularly interesting if you want the agency to have a central account with separate client subaccounts. Its email testing can run up to six variations and automatically send the winner to the remainder of the audience based on opens or clicks. help.gohighlevel.com

Campaign Monitor may actually be the easiest if your agency's primary job is newsletters/campaigns rather than elaborate CRM automation. It specifically provides a centralized way to manage client accounts, campaigns, automations, templates and reporting while keeping each client's account separate. www.campaignmonitor.com

The key distinction

If by "segmentation and split testing" you mean something like:

Client A has 50,000 contacts → create segments → randomly split each segment → test different offers/messages → automatically identify the winner → report results → repeat this across 20 clients

then I'd lean ActiveCampaign or HighLevel, rather than Mailchimp.

If instead you mean website landing-page/conversion A/B testing across many client websites, that's a different category. Convert is built specifically around centralized multi-client experimentation, including separate client projects and permissions. www.convert.com

If you tell me (1) email only vs. email + SMS, (2) roughly how many clients/lists, and (3) whether clients need to log in themselves, I can narrow this down to the 2–3 platforms I'd actually consider and explain the pricing/workflow tradeoffs.

Yes. For a small agency managing multiple client lists, segmentation, and A/B testing, you’ll usually want a platform with:

• separate client workspaces/accounts
• audience segmentation (tags, behaviors, demographics, lifecycle stages)
• split testing (subject lines, content, send times, offers)
• automation/journeys
• client reporting
• ideally agency-level management so you aren’t juggling 20 logins

A few good fits:

A few scenario recommendations:

If you have 5–50 small business clients

I’d look at:

1. Campaign Monitor — easiest agency workflow
2. Brevo — lower cost
3. Zoho Campaigns Agency Edition — good multi-client structure www.zoho.com

If you are doing serious retention/lifecycle marketing

Choose:

• ActiveCampaign
• Klaviyo (especially ecommerce)

If you mean website conversion split testing (not email)

For testing landing pages, funnels, and website experiences across clients, look at:

• Convert.com — it specifically supports multi-client A/B testing workflows, permissions, and centralized experiment management for agencies. www.convert.com

One important distinction: email A/B testing (subject lines, offers, sequences) and CRO testing (landing pages, checkout flows) are usually handled by different tools.

If you tell me:

1. how many clients/lists you manage (e.g., 10 clients × 20k contacts),
2. whether this is email marketing or website testing, and
3. whether clients need their own logins/reporting,

I can narrow it to 2–3 best choices.

Yes. If you mean email/SMS marketing lists, there are a few good fits for a small agency managing multiple clients.

My shortlist

1. ActiveCampaign — probably the easiest overall agency choice.

ActiveCampaign explicitly supports agencies managing multiple client accounts through its Partner Portal. help.activecampaign.comwww.activecampaign.comacademy.klaviyo.comhelp.klaviyo.comacademy.klaviyo.comcustomer.io It also has unusually flexible testing: you can test up to five variations, including email content, subject lines, send times, and entire automation journeys. www.activecampaign.com

The important distinction is that you can keep each client's contacts/account isolated while still managing the accounts as an agency. That's much cleaner than trying to put every client into one giant database.

ActiveCampaign

2. Klaviyo — best if your clients are e-commerce brands.

Klaviyo has a particularly good multi-account model. Its Portfolio feature gives an agency a centralized view across multiple client/brand accounts, including performance, users and billing. academy.klaviyo.com You can also clone segments, flows, forms, campaigns and templates between accounts. help.klaviyo.com

Its segmentation is excellent, and Klaviyo recommends using segments rather than creating lots of separate lists, which tends to make client accounts much easier to maintain. academy.klaviyo.com

Klaviyo

3. Customer.io — strongest if your agency does sophisticated behavioral segmentation.

Customer.io is more data-oriented. You can segment based on essentially any customer attributes/events you feed into it, and its workflow builder supports A/B testing, multi-way splits, random cohorts and branching. customer.io

It also has separate workspaces, so each client can effectively be isolated within the same account structure. docs.customer.io The tradeoff is that it's somewhat more technical than ActiveCampaign or Klaviyo.

Customer.io

What I'd pick

If you're a small agency with, say, 5–30 clients, I'd start with ActiveCampaign unless your clients are predominantly Shopify/e-commerce brands.

If your actual requirement is something like:

"I want to upload/manage 20 client lists, create reusable audience segments, randomly split them 50/50, run tests, automatically pick winners, and see results across clients without becoming an ESP administrator"

then ActiveCampaign is probably the closest fit.

If you tell me what kind of lists you're managing (email, SMS, direct mail, ads, etc.) and roughly how many client accounts/lists, I can narrow this down to 2–3 platforms and compare their agency pricing, client isolation, segmentation, testing, and reporting.

Yes. If you mean email/SMS lists across many agency clients, there are a few good options, but I’d narrow it to these:

My first choice: ActiveCampaign

For a small agency, I'd probably start with ActiveCampaign.

It has unusually strong agency support: its Partner Portal is specifically designed to let agencies/resellers manage multiple client accounts from an account dashboard. www.activecampaign.com

For testing, you can test up to five variations of campaigns, including subject lines, content, CTAs, send times, etc., and automatically send the winning version to the remainder of the list. You can also split-test paths inside automations. www.activecampaign.com

ActiveCampaign

If segmentation is the really important part: Ortto

Ortto is particularly interesting if you're doing sophisticated segmentation for clients.

It lets you create persistent audiences as well as ad-hoc filtered segments, and its segmentation can incorporate customer data and behavior. Its email campaigns support A/B testing of subject lines, sender, content, and send time, with automatic winner selection. help.ortto.com

Ortto

If you have lots of technically sophisticated clients: Customer.io

Customer.io is more powerful but somewhat less "small-business-simple."

The nice thing for an agency is its workspace model: you can have separate workspaces with completely separate profiles, automations, metrics, and data—useful for keeping Client A completely isolated from Client B. You can also copy workflow components between workspaces. docs.customer.io

Its segmentation is extremely flexible, and its workflow builder supports A/B tests, multi-way branches, and random cohorts. docs.customer.io

Customer.io

What I'd choose

If your agency's workflow is something like:

20 clients → each has 1–10 lists → create segments → send campaigns → A/B test → automatically pick winner → report results

I'd go ActiveCampaign first.

If instead you're trying to build a repeatable experimentation/segmentation system across clients, I'd seriously consider Ortto or Customer.io, because they lend themselves better to behavioral segmentation rather than simply "lists."

If you tell me roughly how many clients/lists you manage, whether they're email-only or email + SMS, and whether each client needs a completely separate login/account, I can narrow this down to the best 2–3 and compare pricing + agency management + segmentation + A/B testing.

Yes. If you mean email/CRM lists for multiple agency clients, I’d look first at ActiveCampaign.

My shortlist

1. ActiveCampaign — best overall for an agency

It has a strong combination of segmentation, automation, and testing. You can create segments from tags, behavior, fields, website activity, etc. help.activecampaign.com

For testing, it can do 2–10-way automation splits, percentage-based splits, and winner selection based on defined conditions. Its campaign split testing can also automatically send the winning version to the remainder of the audience. help.activecampaign.com

More importantly for your use case, ActiveCampaign has an agency/reseller Partner Portal for managing multiple client accounts. help.activecampaign.com

ActiveCampaign

2. Brevo — best if you want simpler/cheaper

Brevo is appealing if the agency doesn't need extremely sophisticated CRM automation. It supports segmentation and lets you randomly split segments into separate lists for controlled experiments. help.activecampaign.comhelp.activecampaign.comhelp.brevo.commailchimp.com

Its campaign A/B testing can test subject lines or content, allocate a test percentage, determine the winner, and send the winning version to the rest of the audience. help.brevo.com

Brevo

3. Mailchimp — easiest for relatively straightforward campaigns

If your agency mostly does newsletters and conventional A/B tests rather than complex lifecycle automation, Mailchimp remains very easy to use. It supports testing on particular audience segments, with subject line, content, image, layout, and send-time testing; higher tiers support up to eight multivariate variations. mailchimp.com

Mailchimp

What I'd choose

For a small agency managing, say, 10–50 client lists, I'd lean:

ActiveCampaign > Brevo > Mailchimp

The key distinction is that ActiveCampaign is much more suited to building a repeatable agency process:

Client → audience segments → campaign → randomized test → winner → automated follow-up

rather than treating each email as a standalone campaign.

If you tell me roughly how many clients/lists you manage, average list size, and whether you need email only or email + SMS, I can narrow this down to 1–2 platforms and compare their agency pricing and multi-client workflow.

Yes. If by “segmentation and split testing” you mean managing multiple clients’ email lists, creating behavioral segments, and running A/B tests without a ton of operational overhead, there are a few good fits.

My shortlist

Current agency-focused comparisons tend to put Campaign Monitor ahead for multi-client management, ActiveCampaign ahead for automation, and Brevo ahead on value. hackceleration.com

What I'd pick

For a small generalist agency: ActiveCampaign.
It's probably the best balance if you want sophisticated segmentation such as:

Client A → engaged in last 30 days → purchased X → hasn't received campaign Y → split test subject line → automatically send winner.

It has advanced segmentation, automation, reporting and built-in experimentation, while still being relatively approachable. www.activecampaign.com

For an agency managing lots of relatively independent client newsletters: Campaign Monitor.
I'd lean this direction if your biggest problem is "I have 15 client lists and don't want to constantly fight the software." Agency-oriented testing specifically highlights its multi-client management. hackceleration.com

For ecommerce: Klaviyo.
If most clients are Shopify/DTC brands, I'd choose Klaviyo instead. Its segmentation, behavioral data, flows and revenue attribution are considerably more powerful, although it's more specialized. tooldecisionengine.com

One important distinction

If you're asking about one master audience containing contacts from many clients, I would not recommend that architecture. Keep each client's contacts, sending reputation, domains, permissions, and reporting isolated. Agency platforms should make switching between client accounts easy without mixing the underlying data. ustechautomations.com

If you tell me roughly how many clients/lists you manage (e.g. 5, 20, 50), typical list size, and whether they're ecommerce or lead/newsletter lists, I can narrow this down to the 2–3 platforms I'd actually consider, including likely pricing and how the A/B testing workflow works.

Yes. If by “lists” you mean email/SMS subscriber lists, there are several platforms that are much easier for an agency than stitching together separate accounts.

My shortlist

What I'd pick

For a small agency, I'd start with Campaign Monitor or MailerLite.

Campaign Monitor is probably the closest match if your workflow is:

Client A → import list → create segments → build campaign → split-test → report
Client B → same thing
Client C → same thing
...without having to reinvent your setup every time.

It supports separate client accounts, centralized agency management, permissions, segmentation, automation, templates, reporting, and white-label capabilities. www.campaignmonitor.com

MailerLite is appealing if you prioritize “my team should be able to figure this out without training.” Its agency setup lets one login switch among separate client accounts, while each account retains its own subscribers, campaigns, automations, and templates. It also specifically supports behavioral/demographic segmentation and A/B testing. www.mailerlite.com

Zoho Campaigns is worth looking at if cost and centralized client administration are especially important. Its Agency Edition is explicitly designed around multiple client accounts and includes segmentation, automation, and A/B testing; Zoho currently says you can add up to 10 clients free before scaling further. www.zoho.com

One important distinction

If you mean A/B testing the actual emails—subject lines, content, send time, etc.—the three above are good candidates.

If you mean website/funnel split testing—e.g. 50% of visitors see landing page A and 50% see B—I'd look at a completely different class of tools such as Convert or Varify.io. Both are specifically designed around running experiments across multiple client properties. www.campaignmonitor.comwww.convert.com

If you tell me (1) email vs SMS vs website, (2) roughly how many clients/lists, and (3) whether you need clients to log in themselves, I can narrow this down to the 2–3 platforms I'd actually demo and compare pricing/workflow.

Yes. For a small agency managing multiple client lists, audience segments, and split tests, you’ll generally want something with:

• multi-client workspaces/accounts
• segmentation/tagging
• A/B testing
• automation journeys
• client reporting
• permissions so clients only see their own data

A few platforms fit different agency styles:

A practical agency stack might look like:

Option A — easiest overall

• Campaign Monitor for client email programs
• Google Analytics/Looker Studio for reporting
• Separate client workspaces

Option B — best if you sell marketing automation

• ActiveCampaign
• One account/workspace per client
• Build reusable automation templates

Option C — cheapest way to manage many SMB clients

• Zoho Campaigns or Brevo
• Standardize your segmentation and campaign templates

If by “split testing” you mean email A/B tests (subject lines, offers, send times, content), I’d lean toward ActiveCampaign or Campaign Monitor.

If you mean website landing page/conversion experiments across clients, I’d look more at CRO tools like VWO, Convert, or Optimizely rather than email platforms. www.convert.comvarify.io

A couple of questions would narrow it down:

1. Are these email/SMS lists, website audiences, or ad audiences?
2. Roughly how many clients/lists are you managing (5, 20, 100+)?
3. Do clients need their own login/reporting, or is it internal only?

Yes. If by “client lists” you mean email/SMS subscriber lists across multiple clients, there are several good options. The key distinction is whether you want a true agency/multi-account platform or simply a powerful marketing automation tool.

My shortlist

I'd look hardest at 3

1. Klaviyo — best if you have ecommerce clients

Klaviyo has a dedicated Portfolio system for agencies managing multiple accounts. You can see multiple brands in one place and, importantly, share segments, templates, forms, campaigns and flows between accounts. www.klaviyo.com

That makes it particularly attractive if your agency repeatedly builds similar segmentation/testing frameworks for different clients.

Klaviyo Portfolio

2. Zoho Campaigns — probably the easiest agency-oriented option

This is the one I'd investigate if your priority is “I don't want my team wrestling with a complicated platform.”

Its Agency Edition is explicitly designed around multiple clients, with a central dashboard, separate client accounts, segmentation, automation, A/B testing and reusable templates. Zoho says you can manage up to 10 clients free initially and scale beyond that. www.klaviyo.comwww.zoho.comwww.convert.com

Zoho Campaigns Agency Edition

3. ActiveCampaign — best balance of power and usability

If your agency does more sophisticated automation, I'd lean here. Its split testing isn't limited to subject lines: you can test content, CTAs, send times and even different automation/journey paths, with up to five variations. www.activecampaign.com

ActiveCampaign split testing

One important caveat

If you're talking about website/landing-page segmentation and A/B testing, rather than email-list segmentation, the answer changes. Convert and Varify are much more purpose-built for an agency managing experimentation across many client websites. Convert, for example, has a centralized multi-client dashboard and account/project permissions. www.convert.com

If you tell me roughly how many clients/lists you manage (e.g. 10, 50, 200), whether they're B2B or ecommerce, and whether you need email, SMS, or both, I can narrow this down to the 2–3 platforms I'd actually consider for a small agency.