There are some edge cases, when PHPMailer reports e-mail message as undelivered, but the message itself is delivered successfully to the recipient, causing large amount of e-mail messages delivered until the message record is manually removed from mantis_email_table. As the Mantis e-mail sender reacts to PHPMailer exception as to a delivery failure, there is a possible extension point to increment delivery failures counter and mitigate this issue.

The proposed feature would be opt-in maximum delivery retries (e.g. by specifying number instead of NO for some configuration directive - proposed $g_email_limit_retries, default NO), which will then be checked when e-mail is actually considered undelivered by e-mail sender and in case the number of retries reached the limit, e-mail is deleted using email_queue_delete function and the fact is logged.