Two ServiceDesks in a mail loop

Two ServiceDesks in a mail loop

We provide service to a customer who also uses ServiceDesk.  I'm sure this situation must have come up before, but we're getting mail loops, when our customer forwards a mail from within their ServiceDesk to ours, which creates a request and an acknowledgement email, which in turn creates a new request in their system, and so on.

The only way I can think to stop this is with a multiple line spam filter (on sender and description), but this will still create one incorrect request.

Has anyone come across this before?  Any suggestions on what we can do?