mail has begun to back up in the mail queue for some reason. Attempts to deliver result in;

LOG: MAIN PANIC
Error in system filter: /etc/antivirus.exim does not exist

Any thoughts why this should suddenly be the case and any ideas what to do about it?

I am having this EXACT same problem.

I see your post was back in January. I would like to know if and how you resolved the problem. We are in desparate need of getting email working on this server. I have tried all sorts of ways to resolve the issue short of reinstalling everything.

Take care,
~Gray~

Never did get any response or any clue what the problem was. Eventually had to hire professionals to fix it for me.

antivirus.exim has nothing to do with mail backing up in the queue. You should be looking at the intial installation of Exim if your having email problems. In fact, if you comment out the line in exim.conf dealing with antivirus.exim Exim will continue to function properly without it.