how to detect email loop

Scenario - User creates Outlook rule to forward all email to an external address. For whatever reason, that external mailbox starts rejecting the incoming messages and sends an NDR, which is received by the internal mailbox and then forwarded back out, thus creating a loop on the mail server.

As the Exchange administrator, how can I be notified when this is occurring so that I can disable their mailbox? We recently had an instance that dumped 500K messages in one mailbox.

Thanks!