RBL is an abbreviation for "Real-time Blackhole List". As mentioned below, "RBL" was the name of the first system to use this technology. However, since "RBL" is a trademark for the proprietary MAPS DNSBL, using it as a generic term causes trademark dilution. Some pieces of mail software have configuration parameters that use "RBLs" or "RBL domains" when any DNSBLs can be used, not just the MAPS RBL.