Blocklist

A blocklist is an access control mechanism that lets anyone access a service as long as they are not listed on the blocklist. In the context of email, blocklists usually consist of domain names or IP addresses that have been sending abusive traffic, often spam but also viruses.

There are public blocklists and private blocklists. Public blocklists are open for query by anyone. Some public blocklists charge money for high volume access, in order to cover costs. Private blocklists are not open for query by 3rd parties. Generally private blocklists are maintained directly by an ISP, a corporation or a commercial spam filter.

IP Based Blocklists
An IP based blocklist uses the IP address of the outgoing smarthost to control access. IP addresses that are sending problematic traffic may be listed on a blocklist. Some blocklists, like the [Spamhaus#pbl | PBL] are lists of IP addresses that should not be sending email as a matter of policy.

URI Based Blocklists
URI based blocklists trigger on URLs or domain names in the body of an email to control access. URLs frequently found in spam may end up on both public and private URIBLs.