Opencomputing's OpenProtect is an integrated server side email filtering
solution offering antivirus, antispam and content filtering abilities at the MTA (postfix, sendmail, qmail, exim) level.
With OpenProtect, you can protect your organisation from all email
threats (virus, spam, phishing) and control content (want to filter
password protected zip files? Allow only encrypted emails? disarm HTML
emails and a myraid of other options) at the server level. Available as
GPL/Free software with Commercial support and antivirus plugin.