Which metric is most closely associated with measuring the success of email deliverability?

The metric that is most closely associated with measuring the success of email deliverability is "emails delivered." This metric specifically tracks the number of emails that successfully reach the recipients' inboxes, which is the primary goal of any email marketing campaign. A high delivery rate indicates that the emails are not bouncing back and are being accepted by the mail servers of the recipients.

When analyzing deliverability, it's important to distinguish it from related metrics. While spam score helps identify the likelihood of your emails being marked as spam, it does not directly indicate whether an email was delivered. The email open rate is a measure of how many recipients opened the email, and click-through rate assesses how many clicked on links within the email, but both of these metrics come into play only after successful delivery has occurred. Thus, focusing on the number of "emails delivered" gives a clear picture of how well your emails are reaching their intended audience.