Diagnostic mail for rake redmine:email:receive_imap
As it is in version 1.0, redmine:email:receive_imap doesn't log information when something has gone wrong while handling incoming mail.
It would be a plus to reply to the mail's sender with diagnostic message.
Say the referenced project Id in the mail's body was wrong (tag project: pid). Then redmine:email:receive_imap would send a mail back saying that the referenced project ID was not found in the database.
The mail could be returned to sender or sender + a redmin administrator.