You could have SF listen to the SMTP port as usual and then have SF pass the filtered emails to the listening port of a gateway virus protection like McAfee Webshield SMTP which in turn would pass the filtered cleaned email to the email server port.  So that would now leave the question of does the program you want to use allow you to setup listening ports / forwarding ports?

So it would be more like passing along from product to product in an assembly line.