What is A/B testing in email marketing?

You've written two versions of a subject line and you're staring at them, wondering which one your subscribers will actually open. That's the exact moment A/B testing was made for.

A/B testing (also called split testing) means sending two versions of an email to separate slices of your audience and measuring which one performs better. Version A is your control. Version B changes exactly one thing. You look at the results and let your subscribers tell you which version they prefer.

The "one thing" part matters more than most people realize. Change two things at once and you'll never know which one moved the needle. Subject line and send time in the same test? Your data becomes meaningless.

But Here's what makes a test actually worth running:

• A clear hypothesis. Not "let's try something different" but "I think adding the subscriber's first name to the subject line will increase opens because it feels personal."
• One variable changed. Subject line, CTA copy, send time, preview text, button color. Pick one.
• Enough subscribers. A rough starting point is 1,000 people per variation. Smaller lists can still test, but the results carry more uncertainty. If you've got 500 subscribers total, your best test is still better than no test, just treat the findings as directional rather than definitive.
• A winner metric decided in advance. Are you optimizing for opens? Clicks? Revenue? Decide before you run the test, not after you see the numbers.

So What should you test first? Subject lines are the highest-impact starting point for most senders. They're quick to write, easy to isolate, and they directly affect open rates which affect everything downstream. Once you've got a handle on what makes your audience open, you can move to testing CTAs, content, and send timing.

Most marketing platforms have A/B testing built in. Mailchimp, Klaviyo, and Brevo all let you set up split tests without leaving the dashboard. The difference between platforms is mostly in how they handle statistical significance and whether they auto-pick a winner or leave that to you.

And one One more thing worth knowing: A/B testing doesn't just improve your marketing metrics. Better engagement (more opens, more clicks, fewer ignores) tells mailbox providers your emails are worth showing. That's a real deliverability benefit, not a coincidence. There's a whole thread on how testing connects to inbox placement if you want to go deeper.