The metrics in the list email report that could help the Account Engagement user understand how these prospects may have become unmailable are Total Opt Outs and Hard Bounces (D). These metrics show the number and percentage of prospects who either opted out of receiving future emails or had their emails bounced back due to a permanent error, such as an invalid email address. These prospects are marked as Do Not Email and cannot be emailed again unless they opt back in. The other metrics (A, B, C) are not directly related to the prospects' mailable status, but rather to the email delivery, performance, and tracking. Reference: List Email Report Metrics