In light of the hideous SoBig and a couple of other nasty email based virii, my company decided to implement a virus checker on our Exchange box (before we were depending on client based ones - too easy to switch off!), anyway, small company, expensive products so we wanted to try one out first - I came across the GFI product which is just about perfect for us, they are also offering a freeware version which offers a limited (but still secure) virus checking facility. Anyway, you can get it here and I really do recommend you install it on your Exchange / SMTP gateway (you've nothing to lose and a lot to gain!)