Spam prevention requires preventing open relaying through email
servers. However, legit users want to be able to relay. If legit
users always stayed in one spot, they'd be easy to describe to the
daemon. However, what with roving laptops, logins from home, etc.,
legit users refuse to stay in one spot.

pop-before-smtp watches the mail log, looking for successful
pop/imap logins, and posts the originating IP address into a
database which can be checked by Postfix, to allow relaying for
people who have recently downloaded their email.

Copyright: 1999, 2000, 2001 Bennett Todd
2002, 2003, 2004 Wayne Davison