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 the MTA, to allow relaying for people who have recently
downloaded their email.

WWW:	http://popbsmtp.sourceforge.net/