What is the second main theme in creating high-performing emails, after selecting the right goal?

Optimizing each part of the email for engagement is fundamental to ensuring that the email resonates with its recipient. Once you have a clear goal for your email campaign, focusing on optimization allows you to enhance various elements such as the layout, call-to-action (CTA), copy, and overall design. Each section of an email must work cohesively to guide the reader toward the desired action, whether it's clicking a link, making a purchase, or subscribing to a newsletter.

Improving engagement involves using analytics to understand what elements attract the audience's attention and drive interaction. By adjusting factors like the content structure and visual appeal, businesses can create emails that not only catch the recipient’s eye but also prompt them to engage effectively.

The other options, while relevant to email marketing, do not encompass the broader scope of optimizing multiple aspects of an email for overall engagement. A catchy subject line is important, but it is just one piece of the puzzle. Using images can enhance the message, but without overall optimization, even great visuals can fail to engage if the email's structure or content does not resonate well. Sending emails at a high frequency doesn’t guarantee engagement; instead, it is the quality and relevance of the emails that truly matter.