qSheff is a wrapper for the qmail queue that scans email for viruses and
spam.  Infected messages are rejected before they reach the queue, so the
server doesn't perform any job for them.  After checking the message, it
will wake the qmail queue.  Some of the supported features:

  - Antivirus executing (ClamAV)
  - Header and body filtering
  - Subject filtering
  - Attachment blocking
  - Quarantine support
  - White/black lists
  - Single line logging for qmail
  - Basic DoS attack prevention
  - And much more...