How to A/B Test Your Email Campaigns Properly

Subject lines typically have the largest impact on open rate and are the highest-priority element to test first. Once subject lines are dialed in, test send time, from-name, and call-to-action wording or placement — testing in order of expected impact rather than everything at once.

Test with a large enough sample size and let the test run long enough to reach significance — calling a winner after only a handful of opens produces an unreliable result that's really just noise, not a genuine insight to build future campaigns on.

Change one variable at a time; testing subject line and send time simultaneously makes it impossible to know which change drove the difference in results. Most email platforms have built-in A/B testing tools that handle the split and significance calculation automatically — use them rather than manually splitting lists.

A winning pattern (a specific subject line style, a particular send time) often generalizes across future campaigns, not just the single test — document findings and apply them forward rather than re-testing the same question repeatedly.