Feature Request: add mail header to prevent auto replies like out of office

## Summary Sentry sends emails with the sender address configured in config.yml - mail.from. This mailbox receives all auto reply mails like out of office notifications, which are not very useful in this context. Can you add some mail headers to prevent auto replies on mail notifications? As far as i know there are basically three different headers which most auto reply filters look for: X-Auto-Response-Suppress: All Precedence: Bulk Auto-Submitted: auto-generated ## Motivation This feature would reduce unwanted mails in the mailbox defined by mail.from.