Use third-party security software: ClamXav
ClamXav is based on the GPL ClamAV anti-virus engine that's for detecting and eliminating Trojans, viruses, malware, and other threats. ClamAV claims it's the standard tool for mail gateway scanning. ClamXav in a nutshell:
- Free virus definitions, updated almost daily.
- Crashed occasionally in my tests.
- Works with OS X 10.5 through 10.8 (Mountain Lion).
- Good documentation but not much in the way of support.
- Gives you the choice to turn off background scanning for times you prefer not to slow down your work.