"If you are administrating a mail server and use blacklists to
block spam (like in this article: How To Block Spam Before It
Enters The Server (Postfix)), you probably know this problem: from
time to time your customers complain that they cannot receive
emails from certain freemailers. Most often this happens because a
freemailer was abused to send out spam and therefore got
blacklisted. This short guide shows how you can whitelist such a
mail server in Postfix to make your customers happy again..."