Exchange Hop count exceeded - possible mail loop

Exim was used as a relay in front of Exchange 2010. Exchange had a mail contact, not a mailbox, with the address [email protected]. No transport rules were configured for this contact. The domain.com Exchange domain was configured as authoritative: email is delivered to recipients in this Exchange organization.

Situation: a message arrived from [email protected] for [email protected]. Exim successfully relayed it to Exchange, and Exchange accepted it. Exchange then immediately sent it again, using [email protected] as the sender, to [email protected]. It returned to the relay, which delivered it to Exchange again. The loop continued until this error appeared:

SMTP error from remote mail server after end of data: 554 5.4.6 Hop count exceeded - possible mail loop

Solution: remove the contact. Since it no longer forwards mail anywhere, it is no longer needed.