Emails the are replies to Digest emails, are getting rejected. Currently They are rejected by
BadDestinationError. This is the catchall, that multiple cases result in.
This PR adds a check to make sure sure that the
In-Reply-To header of incoming messages does not match the
message_id of a Digest email that was sent out. If this is the case, we can send a much more specific error email, saying that you cannot reply to a digest email.
Email debugging is fun