Clam AntiVirus (ClamAV) is a Linux based, anti-virus toolkit for mail servers with automatic updating via the Internet. ClamAV is very fast, secure and supports compressed files - so .zip email attachments will be safely scanned.
Real Time's current virus-scanning solution scans both incoming and outgoing email messages. This is to block email viruses from other Internet users; and prevents any local infections from spreading out to the Internet, which might clog your bandwidth and/or cause your service to be suspended.
Since ClamAV is Open Source, there are no site license fees or per user fees to use it.