What is primarily measured in the success of email campaigns?

When evaluating the success of email campaigns, it is essential to consider a range of metrics that collectively provide insight into the campaign's performance. Open rates measure how many recipients opened the email, which indicates the effectiveness of the subject line and the overall interest in the content. Subscriber numbers reflect the size of the audience, showing potential reach and effectiveness in attracting new subscribers. Click-through rates gauge how many recipients engaged with the content by clicking on links, signaling the email's ability to drive action and foster engagement.

Given that each of these metrics provides valuable information about different aspects of an email campaign's performance, assessing all three together gives a more comprehensive understanding of success. Therefore, the option that encompasses all these measures as contributors to email campaign success is the most accurate.