Routine SMTP errors like invalid recipients pause MailService for 5 or more seconds

Invalid recipients or senders for emails cause the MailService to pause for 5 seconds (by default) as this is caught as a general `MailException` and not differentiated from a network failure etc.

In systems where recipients mailed to are frequently invalid (e.g. an employee leaving but remaining on several active PRs) this can degrade mail throughput especially for systems at scale.