Quote:
On a few computers I checked the virus scanner logs and viruses were removed from computers by the virus scanner as the email was downloaded.

You didn't find these in a scane for exactly this reason - the firewall removed them from the email before they even made it to the machine. Which means that someone is sending you infected emails, you're downloading them off the mail server, and the email virus scanner is deleting them. That's about as good an outcome as could be expected. If you know which emails are infected you could try and notify the person sending them, but the likelyhood of that working is next to zero.

Matthew