A/B testing removes guesswork from marketing decisions. Instead of debating whether Subject Line A or Subject Line B will perform better, you send both to a sample of your audience and let the data decide. The winning version goes to the rest of the list.
In email marketing, the most commonly tested elements include subject lines, sender name, send time, CTA copy, email layout, and hero images. On landing pages, teams test headlines, form length, CTA button color and copy, social proof placement, and page layout.
For reliable results, A/B tests need sufficient sample size and a single variable. Testing the subject line and the CTA simultaneously means you cannot isolate which change drove the result. Change one thing per test. If your list is small, focus on testing elements with the biggest potential impact (subject lines affect open rates more than font changes).
Most MAPs have built-in A/B testing. Marketo offers Champion/Challenger for email programs. HubSpot supports A/B testing in email and landing pages. Salesforce Marketing Cloud has A/B testing in Email Studio. For landing page testing beyond your MAP, tools like Optimizely, VWO, and Google Optimize (sunset, but alternatives exist) provide more sophisticated capabilities.
The discipline of A/B testing matters more than any individual test result. Build a testing cadence. Run at least one test per month on your highest-volume email programs. Document results. Over time, those incremental improvements compound. A 5% improvement in open rate plus a 10% improvement in click rate plus a 15% improvement in landing page conversion adds up fast.