Trend Micro is another familiar name in the anti-virus world. It has now branched out into email security with its InterScan Messaging Security Suite (IMSS), which, unsurprisingly, places a heavy emphasis on virus containment, but which also contains a number of other extremely valuable features.
IMSS is another spool-and-forward product, which means that the machine it is installed on should have a hard drive sufficient to handle your usual throughput of email. For this reason, and for reliability, it shouldn't really be installed on the SMTP server. Depending upon the needs of your network, it can be installed either in front of or behind your existing firewall, or even used in place of a firewall if - for some strange reason - you don't actually have one.