A/B testing, a crucial aspect of email marketing, involves sending two versions of an email to different groups of recipients to measure the impact of changes on performance. It helps marketers identify which specific elements of their email work best and by how much. A/B testing can be applied to various aspects of emails, including subject lines, message previews, plain text, salutations, body copy, images, design and layout, call-to-actions, delivery day and time, and more. By setting up an A/B test, marketers can gain valuable insights into what works best for their specific audience, refine their email campaigns, and continuously improve their marketing efforts.