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.