Email A/B testing is the process of sending two versions of an email to a portion of a list to see which one performs better, then sending the winning version to the remaining subscribers.
The two versions, usually labeled A and B, differ by a single variable, such as the subject line, send time, or call-to-action button. The version that gets more opens, clicks, or conversions is considered the winner and is sent to the rest of the list. Split testing email campaigns this way takes the guesswork out of what an audience prefers, since the decision is based on actual subscriber behavior rather than a hunch.
For example, an online store might test a discount-led subject line against an urgency-led one to see which drives more opens before a sale campaign goes out to the full list. Running an email marketing A/B test like this is one of the simplest ways to improve results without any added cost, since the same winning version can be reused on future sends.
What Is A/B Testing in Email Marketing?
At its core, it is a controlled comparison. One group of subscribers sees Version A, another group sees Version B, and whichever version performs better on a defined metric, such as open rate or click rate, becomes the version sent to everyone else. Because the results apply to every future send using the winning version, even a small improvement compounds over time.
How to A/B Test Subject Lines and Other Elements
- Choose one variable to test. This can be the subject line, send time, hero image, or CTA button. Testing more than one variable at a time makes it impossible to know which change caused the result.
- Split a sample of the list. A common approach is testing on 20 to 40 percent of subscribers, split evenly between Version A and Version B.
- Let the test run long enough. Ending a test too early risks reacting to random variation instead of a real pattern. The sample needs enough opens or clicks to produce a reliable result.
- Send the winning version to the rest of the list. The remaining subscribers automatically receive whichever version performed better, a step most email marketing platforms automate.
What to Test First
For most stores new to email A/B testing, subject lines are the best place to start, since they have the largest effect on whether an email gets opened at all and require no changes to the email body or design. Once subject line testing becomes routine, testing send time, CTA wording, and layout are logical next steps.
Why Most Stores Are Still Leaving This on the Table
Despite how much A/B testing is recommended, actual adoption remains surprisingly low:
- Only about 39 percent of email marketers regularly A/B test subject lines, per Q1 2026 industry benchmark data across Mailchimp, Klaviyo, and HubSpot, meaning most campaigns still go out without testing the single variable known to move performance the most.
- Up to 49 percent higher open rates for businesses that do test consistently, compared to sending untested, according to reports from multiple email platforms.
That gap between how well-known the practice is and how rarely it’s actually done consistently is where the real opportunity sits.
Related terms:
Adflipr’s Automations feature includes Split Path, which lets you branch subscribers down different paths inside a workflow so you can compare what performs better, without building the logic manually.
