A/B testing is a popular method used by online marketers to test different variations of a given message. For email marketing, the most common scenarios of A/B testing are to test two or more subject lines (i.e. the title of the email) and to test different calls to action (e.g. text or image call to action). By sending the same email but with slight variations of it, you can figure out which email is the best performing one in terms of open rate or click through rate.