Acronym for Realtime Black List

a list of hosts you may wish to blacklist updated in realtime, usually distributed via DNS. Realtime blacklists originally were created as lists of open relays, so that people could block spam that was being delivered via them. This gradually grew to include blacklisting hosts for other reasons (open proxies, sending spam directly, political views oppose your own).

Mailservers then are configured to check a RBL everytime they recieve a mail connection to determine if they should drop the connection.

RBL's are usually done by looking up ip.rbl.somedomain.com and seeing if it contains an A record (usually 127.0.0.2) and/or a TXT record (explaining why it's banned).

Many people subscribe to multiple RPL's.