A blacklist is a list of IP addresses or domains that have been reported as known sources of spam.
There are two types:
- Public blacklists are made available to the public. Some are offered as a free service, while others require a fee. There are hundreds of well known public blacklists, and there may be thousands in existence. The number of blacklists can change at any time, because many blacklisting systems allow mailbox providers and system administrators to start their own list based upon another list.
- Private blacklists tend to be based on strict spam filtering and cannot be queried externally. You will not know you are on a private blacklist until you receive bounce messages from the spam filters. The bounces are usually managed by the mailbox provider.