Filtering E-mail for Viruses using Linux?

Cliff Hammerschmidt asks: "I'm looking for a package to scan incoming and outgoing e-mails for viruses. I'd like to do it on the Linux server that handles the e-mail. Nice features would include automatically responding to infected messages to let the sender know they are infected, the automatic cleaning of viruses from attachments, and automatic updating of virus data." Two and a half years ago it was safe to simply open your e-mail, regardless of content. Now thanks to the wonders of integration, e-mail viruses exist. How reliable are the programs that test for these new critters?