At work I use Ipswitch's very capable Imail mail server.  A few years ago, as nasty things were increasingly delivered via e-mail, I started investigating an anti-virus package to run on the mail server.  Afterall, if the nasties never get to the client machine, all the better.

At the time (v6.x), Ipswitch did not offer an anti-virus package, but they did tell you how to tie an external filter program into Imail.  This allows you to parse and process messages before they are sent to local or remote users.

Computerized Horizons (dba: Declude) makes some very nice, well thought out products that (I believe) hook the Imail system in this very way to do anti-virus and anti-spam.  If you're not someone like me, who likes to build and tinker with their own projects, you would do yourself (and your company) a favor by checking them out.

If you prefer to be more intimately involved in these programs on your system, then I will make the source code available for myImailAV. Please e-mail me with inquiries.