Which metric indicates the percentage of emails that could not be delivered?

The bounce rate is the metric that indicates the percentage of emails that could not be delivered. This metric is crucial for email marketers as it reflects the effectiveness of their email campaigns and database quality. When an email bounces, it means it was not successfully delivered to the recipient's inbox. Bounces can be categorized into two types: hard bounces, which occur when the recipient's email address is invalid or does not exist, and soft bounces, which happen when there are temporary issues such as the recipient’s inbox being full.

Understanding the bounce rate helps marketers identify and address potential issues with their email lists, such as removing invalid addresses to improve future delivery rates. This metric is directly related to the overall health of an email campaign.

The other metrics listed pertain to different aspects of email marketing. The open rate measures how many recipients open the email, the click-through rate indicates how many clicked on a link within the email, and the unsubscribe rate tracks the percentage of users who opted out from receiving future emails. Each of these metrics provides valuable insights, but they do not provide information about delivery failures, which is why bounce rate is the correct choice in this context.