Definition

A while back, some server administrators got really angry at all the junk mail they received. So they started to track the IP addresses of the servers that sent them spam and put them on “blacklists.” Anytime they received email from a server on their blacklist, it was deleted immediately. It worked pretty well. So they started to share their lists with other server administrators. And other admins started to add on to those blacklists. Pretty soon the blacklists got very, very big. Large ISPs started to sync up with them. If you send email that’s very “spammy” you could end up on one of these blacklists (whether the recipients are opt-in or not). Once you make it on to a blacklist it’s a lengthy process to get off. This is why you need to make sure you only send to recipients who gave you verifiable permission to email them and you need to have proof that each one of them opted in to your list (such as through a double opt-in system).