A blocklist is a list of IP addresses or domains exhibiting characteristics of spam. Mailbox providers and spam-filtering companies use a blocklist to identify IP addresses and domains suspected of sending spam to aid their filtering decisions.
There are two types:
- Public blocklists 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 blocklists, and there may be thousands in existence. The number of blocklists can change at any time, because many blocklisting systems allow mailbox providers and system administrators to start their own list based upon another list.
- Private blocklists tend to be based on strict spam filtering and cannot be queried externally. You do not know you are on a private blocklist until you receive bounce messages from the spam filters. The bounces are usually managed by the mailbox provider.