Acronym for Realtime Black List

A constantly updated list of hosts sending Email you may wish to reject. Originally created to list open relays abused to send Spam, there are now various RBLs which also blacklist hosts for reasons such as being an open Proxy, sending Spam directly, or having political views that oppose your own.

RBLs are usually queried via DNS, by looking up Lookups for blacklisted hosts return an A record (usually for and/or a TXT record (explaining why the host is blacklisted).

MailServers can be configured to check RBLs to determine whether they should accept an incoming SMTP connection. Most people subscribe to multiple RBLs.