SMTP: Advanced Queue failed to deliver message

Hi all,

Having a problem with someone sending an e-mail to one of our users. Using
Windows server 2000. Some has sent an e-mail and have had a NDR saying:

"Unable to deliver message to the following recipients, because the
message was forwarded more than the maximum allowed times. This could
indicate a mail loop."

This was sent from our postmaster.

The sender then recieved an e-mail from his postmaster saying:

Your message has been relayed
to the following recipient(s):

rSim@(domain).com
Message relayed


The message was successfully delivered to 1 recipient(s) at domain
xxxxxxxx.com.

However when I check the Exchange system manager message tracking they come
in but get this message:

SMTP: Advanced Queue failed to deliver message
SMTP: NDR generated

But the main thing I noticed was that another sender which has the same
e-mail ending, @cbre.com was also getting the same NDR.

Can anyone confirm if this is a problem with our server or their end? What
do I do to sort this?

Luke