Just thought I'd like to share our spam and virus management method.
We use Microsoft Exchange Server 2003 (We will upgrade to 2007, when I have the energy - each time we mess with Exchange Server it costs me a night of sleep), and found that even using the built-in Intelligent Message Filter and connection filtering using RBL's (SORBS, SpamCop, et al), we still got heaps of spam (95:5 spam ratio).
Recently I installed Microsoft AntiGen (A recent Microsoft Aquisition), and found that since doing so, our spam levels dropped close to 1%.
So, if you (or your customers), are using Microsoft Exchange, and have a spam problem then this is worth trying out!