This is one cool application, I'd rate this right up there with Apache and Samba as a killer Linux app. For a while I'd been looking for an intergrated spam/virus solution. I'd previously been using Mail Monitor from Sophos for the virus element. For spam I've tried spamassassin from procmail, and as a daemon using amavisd. While this all worked, it wasn't what I'd describe as an ideal solution. I ended up writing a web interface to mail monitor to manage the quaranted messages, but it was still more effort than I wanted to be spending (the point of scanners is to reduce effort yes?).
But then I found Mail Scanner, and life is sweet :). What can I say, Mail Scanner intergrates spam assassin and many existing commercial and open source virus scanners. Mail Watch then adds a web interface, which gives a number of graphical reports as well as allowing the managment of quarantined messages.
Screenshots (shamelessly ripped from http://mailwatch.sourceforge.net)