A/B testing, also known as split testing, is a method used to compare two versions of an email to optimize its content, design, or other elements for more effective results. In E-Mail Marketing, A/B testing can be used to test subject lines, email body text, images, call-to-action buttons, and other elements. By analyzing the different responses from recipients, marketers can determine which version is more successful in achieving desired goals, such as open rates or click-through rates.