Email marketing tool with solid A/B testing of subject lines

Klaviyo is my top pick if you want strong, flexible subject-line testing. It lets you test multiple subject lines, choose a winning metric, and—when you send the test to only part of your audience—automatically send the winner to the rest. It also supports subject-line testing in both campaigns and automated flows. (help.klaviyo.com)

Other good options:

• ActiveCampaign — a solid choice if you also want automation. Its split tests cover subject lines and sender details, and can send the winner to the remaining contacts. (help.activecampaign.com)
• Mailchimp — straightforward for basic campaign testing; you can compare subject lines and other variables like sender name, content, or send time. (mailchimp.com)

Quick recommendation: Klaviyo for more sophisticated testing; Mailchimp if you mainly want simple, easy-to-use subject-line tests. If you tell me your list size, budget, and whether you sell online, I can narrow it down.

Short answer: For subject-line testing, Mailchimp, ActiveCampaign, and Klaviyo are the strongest general options. Choose by your sending style and budget.

• Mailchimp (general newsletters and small teams): Its testing covers subject line, from-name, content or send time, with up to three variations, and winners can be picked by opens, clicks, or revenue, automatically or by hand. Its free tier is limited, and A/B testing starts on paid plans.
• ActiveCampaign (more complex, automation-heavy setups): It allows automation split testing, which routes contacts between two entire automation paths, so you can test more than the subject line. Its campaign tests support up to 5 A/B variations.
• Klaviyo (ecommerce): It tests 4+ variations, flows and picks winners automatically after a sample. Its flow tests run continuously as contacts arrive, which smooths out day-of-week and hour-of-day variance.
• Brevo (budget option): Its testing is focused on subject lines, and A/B testing is included from the Standard plan. Note that Brevo recommends at least 5,000 recipients for statistically relevant results.
• Omnisend (ecommerce on a budget): Testing is available on all plans, including the free tier, and it tests subject lines and sender names.
• HubSpot: Only worth it if you already run HubSpot's CRM, since it offers 2 variations only and is priced high.

Tips for reliable subject-line tests:

• Sample size matters. One source suggests a result only means something at roughly 1,000+ recipients per variant.
• Judge winners on clicks or replies rather than opens, since Apple inflates opens.
• Prices and plan limits change often, and sources I found disagree on some figures, so confirm current pricing on each vendor's site.

If you tell me your list size, whether you're ecommerce or content-focused, and your budget, I can narrow this to one recommendation.

For robust subject line A/B testing, the best approach is to use the built-in testing features already included in your Email Service Provider (ESP). Since these platforms handle the actual sending, they are best equipped to split your audience, track results, and automatically deploy the winner.

Recommended Tools by Use Case

1. Best All-Rounders (Built-in A/B Testing)

These platforms are designed to handle marketing newsletters and campaigns with ease. They allow you to test multiple subject lines, content, and send times.

• ActiveCampaign: Highly recommended for its advanced testing capabilities. It allows you to test up to five variations (A/B/C/D/E) simultaneously and uses "Active Intelligence" to automatically send the winning version to the remainder of your list.
• Mailchimp: A great entry point for smaller businesses. It offers straightforward A/B testing, including subject line variations, and is accessible even on lower-tier plans.
• MailerLite: Often cited as a top choice for those needing a balance of features and affordability. It includes A/B testing features even on its free plan, making it excellent for bootstrapped companies.
• HubSpot: Ideal for larger organizations that require deep CRM integration and want to perform complex multivariate testing within their automated workflows.

2. Best for Cold Outreach

If your primary need is "cold" sales outreach rather than marketing newsletters, standard ESPs may not be the right fit. Instead, use sequence-based tools designed to handle deliverability for individual outreach.

• Tools: Instantly, Smartlead, and Lemlist are the industry standards here. These tools are built to test variations across your entire follow-up sequence, not just the initial subject line.

How to Get Reliable Results

Regardless of the tool you choose, follow these best practices to ensure your tests are meaningful:

• Avoid "Thin" Testing: A/B tests on open rates are only statistically reliable if you have a sufficient audience size (generally 1,000+ recipients per variant). With smaller lists, you may need a larger time window or a more significant difference in performance to see a "real" winner.
• Test One Variable at a Time: To know exactly what drove your results, only change one element (e.g., subject line) while keeping the email body identical.
• Prioritize Clicks Over Opens: Due to privacy features like Apple’s Mail Privacy Protection, "open" metrics can be inflated. If possible, judge your A/B test success based on clicks or replies, which are more concrete indicators of engagement.
• Use Automation for Winners: Look for tools that offer "automated winner selection". This saves you from having to manually check results and restart campaigns, as the tool will automatically identify the best performer after a set period or sample size and send it to the rest of your audience.

Klaviyo is a strong pick if you want more than basic subject-line split testing. It supports tests in both campaigns and automated flows, can compare multiple variations, and offers AI-generated subject-line variations. ¹¹⁰

If you want a straightforward option for general-purpose email marketing, Mailchimp lets you test subject lines and automatically send the winner to the rest of your audience. A/B testing is available on Essentials and higher. ⁴⁹

Quick take: choose Klaviyo for testing across campaigns and flows; choose Mailchimp for simpler campaign-level testing.