Just have a look at:
http://www.ravantivirus.com I had installed the same but could not give it a go coz of my lower version of Courier-MTA. Commercially, a 6 domain license would cost 450 USD as offered to me and the updates are all run as a daemon process. The AV is available for following Mail Servers and OS combination:
MS Exchange 5.5
MS Exchange 2000
I am looking for someone to upgrade my courier-mta to the latest version and then again I'll have a go. They offer free testing on two domains for 60 days and installation from commandline is just two minutes job at the most.
PS: Sorry for the formatting. I am not sure how to get the table tags up on Forum....