Debian-Administration has another massive tutorial, this time on the relevant issue of having SPAM/Virus Filter on Sarge
It's long, it's here
This documents my adventure of setting up a spam/virus fighting mail server on Sarge. It is not the most fancy way of doing it, but should be suitable for an environment with not too many users, nor a bunch of road warriors. It's also probably a decent starting point for building more advanced configurations. The end product is a server that dicards virus-infected mail, identifies spam, and sorts it into a local user's junk folder.