Has anyone out there run into the issue with mail going back to a spammer as "User unknown"?

We have one relay server that handles both in/out messages to the internet. Looking at the mail que I see tons of messages that are being bounced back to spammers as "User unknown". Which in turn flood the mailq. So any legit email that is trying to come in or out gets delayed.

I am monitoring the mailq via snmp, using a thrid party montioring utility. Then when I get the alret just clear out the mailq.

Doing:

grep -l "User unknown" * > file
then
for i in `cat file;do rm ??$i;done

This is a pain! Is there something I can do?

Help!!!!