Email Loops back with errors

Were currently getting the following error when trying to send a message to a specific client. I’ve search the internet and came up with nothing.

here is a copy of the error message:
“This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following addresses
failed:

address generates mail loop:”

Currently running Outlook 2003 on Exchange 2003
There’s no rules or alerts nor out of office assistance applied to the users outlook profile

if more info required please let me know.

Thanks in advance.