…77% of the time at least.
Synthesys’s mail server runs Postfix + ClamAV via clamsmtp, and I haven’t received an e-mail virus since setting it up. Evidently this isn’t just a fluke or good luck because Electric Mail found that 77% of the time ClamAV was the first antivirus product to block new viruses.
If you’re not using ClamAV to scan your e-mails, you should. And if you have some spare change, support the good guys.