Email handlers should send email back upon rejection.

If an error occurs when a comment handler is processing an email message, the message is left in the email spool and will continue to generate an error message in the logfile. A typical error that we envision is a user sending an email from a different email account where the return address doesn't match the user's address in the database.

The problem is that nobody is notified that the error has occurred and the comment is lost until someone stumbles across it. It would be better if an email was sent back to sender indicating that an error occurred.