"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..."
