I use AntiVir Mailgateway from H+BEDV on a big corporate systemt. It has good documentation and som great features. Updates at least once a day.
I believe AntiVir is free for private and non-commecial use, but you have to pay for it otherwise, wich is fair, since these guys are putting a great deal of work into it.
Check it out here:
http://www.hbedv.com/
Regards.