Dynamic Relay Authorization Control (pop-before-smtp)
A daemon that dynamically updates a relay authorization map for some
MTA (postfix, sendmail). 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 different hosts.
http://mail.cc.umanitoba.ca/drac/
