RBL (real-time block list)

A database of Internet Protocol (IP) addresses of sources that have been verified as being unsolicited commercial e-mail (also known as spam). E-mail messages from certain e-mail addresses or domains can be blocked by adding the sender to your Blocked Sender list in Microsoft(r) Office Outlook(r). [Exchange 2003]

A dynamic list that is maintained inside an organization or by a remote third-party organization that contains a list of known unsolicited e-mail or spam sources. The receiving mail server may refuse connections from addresses that are on a RBL. [Exchange 2007]