1. 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. From Debian 3.0r0 APT