What elements can be A/B tested? (Subject lines, CTAs, content, send time)

You've sent dozens of campaigns and you still wonder: would a different subject line have doubled my opens? Would a red button have outperformed the blue one? That's exactly what A/B testing is for. But before you test everything at once, it helps to know what's actually worth testing and in what order.

The short answer is that nearly every element of an email can be tested. Here's how to think about them by impact.

Start here: the highest-impact tests

Subject lines are the most common starting point, and for good reason. You can test length (short vs. descriptive), tone (curious vs. direct), personalization (first name vs. none), urgency (deadline vs. no deadline), and whether an emoji helps or hurts. Opens are easy to measure, and even a small sample size can show a real difference.

Preheader text pairs with the subject line in every inbox preview. Most senders ignore it entirely, which makes it one of the easiest wins to find. Test whether a teaser, a call to action, or a continuation of the subject line gets more opens.

Sender name affects trust before the email is even opened. A personal name ("Yanna from RME") often outperforms a brand name ("Review My Emails") for certain audiences. Worth testing once per list segment.

Inside the email

CTAs are where clicks live. You can test button text ("Get started" vs. "See the plan"), button color, placement (above the fold vs. after copy), and whether one CTA or two performs better. These tests are clear to measure because clicks are a hard number.

Body copy is trickier but worth it for high-volume sends. Test long vs. short copy, a storytelling approach vs. bullet points, or a direct offer vs. a soft lead-in. One element at a time, always.

Images can swing results significantly depending on your audience. Test product photography vs. lifestyle imagery, or no image at all. Some lists respond better to plain-text-style emails. You won't know until you test.

Layout and design changes like single-column vs. multi-column, or heavy visual vs. minimal, are bigger lifts to set up but worth running if your design hasn't changed in a while.

Timing and frequency

Send time and day matter more for some industries than others. A B2B audience might open Tuesday mornings. A consumer list might spike on Sunday evenings. Test them, but know that timing tests need larger sample sizes to produce reliable results because the variance is high.

A practical testing order

Don't try to test everything at once. A good sequence is to start with subject lines (they're fast to run and easy to read), then move to CTAs, then body copy, then layout. Each test should change one thing only. If you change the subject line and the CTA in the same send, you won't know which one moved the needle.

You'll also want to make sure you have enough recipients per variant to trust the result. A test with 200 people per variant might show a difference, but it's hard to know if it's real or just noise. If you're not sure what counts as a reliable sample for your list size, our next question covers how to run a statistically valid A/B test without needing a statistics degree to understand it.

And if higher engagement from better-tested campaigns is your goal, it's worth knowing that A/B testing also improves your deliverability. More opens and clicks tell inbox providers your emails are worth showing.

If you're using an ESP like Mailchimp, Klaviyo, or Brevo, A/B testing is usually built in. Start with a subject line test on your next campaign and go from there.