Where can I find the A/B testing feature?
The A/B testing is a separate feature which can be found under Tools > Email marketing > A/B tests.
There you can create a special message with the A/B testing feature where you can configure your test.
Legacy editor
The A/B Test feature is a part of the newsletter creation process. Whenever you create a newsletter message, you will find it at the bottom of the first page, in the Distribution settings section.
Turn on the A/B Test switch to enable it. Once enabled, the newsletter creator will be extended with additional steps where you define the type of test and its settings.
If you wish to disable the A/B Test simply go back to the message settings and switch the test to OFF.
