Description:

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.