If you're using NAV CE for virus checking, you can use NAV gateway to not only check incoming/outgoing emails/attachments for virus' but also limit attachment size, or strip off attachments with certain suffixes (i.e. strip/disallow any attachment with a suffix of .vbs, .exe, etc) as well as disallow emails with certain keywords (disallow any email with a subject containing "xxx", etc).
So far it's working perfectly.
Mark