Server Side Virus Scanning Options?

Unknown Relic asks: "Because of the number of virii which are propagated through email, and the tendancy for some users to open executable attachments no matter what they are told, we have decided to seek out a server side solution. We are currently running Linux with qmail on the server side, and while a we have found a couple of products which may fit the bill, I wanted to hear about the experiences and recommendations of slashdotters on this subject. Do you or your company make use of a server side virus scanning engine, Open Source or otherwise, and if so what are your impressions?"