CleanMail - Attachment Filter
Blocking malicious attachments
Most worms and viruses are spread by attachments. Getting rid of messages containing potentially malicious attachments before even starting virus checkers or spam filters can be very helpful to reduce system load.
CleanMail's built-in attachment filter allows you to specify a list of attachments that you want to accept, or, the other way around, you can specify a list of attachments you do not want to accept. The attachment filter is pre-configured to reject all attachment types known as potential virus vectors.
The attachment filter is also capable of detecting MIME violations. MIME violations can disrupt mail server operation and sometimes crash mail clients. Also, worm authors could try to hide executable attachments with deliberate MIME syntax violations.