It's a lightweight, fast and reliable Sendmail milter that implements
a real-time Sender e-Mail Address Verification technology. This technology
can stop some kinds of SPAM with a spoofed sender's e-Mail address.
  Also it implements a real-time Recipient e-Mail Address Verification
technology. It can be useful if your machine is a backup MX for the recipient's
domains or if your machine forwards all e-Mail messages as a relay host for your
domains to another internal or external e-Mail servers.
  It's a lite alternative for the spamilter, milter-sender and milter-ahead
milters.

WWW: http://smfs.sourceforge.net/