Issue Summary

Currently, if an outbound e-mail (e.g. a notification) fails to be sent (e.g. due to a transient SMTP error), that e-mail is discarded and never sent again.

This may cause important notifications to be missed by the recipient.

Desired behaviour

Bitbucket Server should re-enqueue outbound e-mail that failed to be submitted to the SMTP server and retry sending these. The number of retries and how much time should pass between them could be configurable.