ChatGPT answers, unedited

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. 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 “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. 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.