clamav

The ClamAV scanner is available for many operating systems and detects several malware. In order to configure Postfix to reject a message during the SMTP delivery a pre-queue filter is needed. clamav-milter can be configured as a pre-queue filter, forwards messages to clamav and sends its feedback back to postifx.

Detailed steps:
install clamav and clamav-milter
configure clamav-milter to "reject" infected messages
tell postfix to use the clamav-milter socket as a smtpd_milters