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 ip.rbl.example.com. Lookups for blacklisted hosts return an A record (usually for 127.0.0.2) and/or a TXT record (explaining why the host is blacklisted).