DRAC is a daemon that dynamically updates a relay authorization map for
Sendmail, Postfix and other MTA's that supports it. It provides a way to allow
legitimate users to relay mail through an SMTP server, while still preventing
others from using it as a spam relay. User's IP addresses are added to the map
immediately after they have authenticated to the POP or IMAP server. By
default, map entries expire after 30 minutes, but can be renewed by additional
authentication. Periodically checking mail on a POP server is sufficient to do
this. The POP and SMTP servers can be on a POP server is sufficient to do this.
The POP and SMTP servers can be on different hosts.

WWW: http://mail.cc.umanitoba.ca/drac/