RBL

What it means

RFC 5782 tells the history. In 1997 Dave Rand and Paul Vixie kept a list of IP addresses that had sent them spam or behaved objectionably, first distributed as a BGP feed for routers. It became known as the Real-time Blackhole List (RBL). Many network managers wanted it for email but not as a BGP feed, so Rand and Vixie created a DNS-based distribution that quickly became more popular, and others built their own DNS-based lists on the same model.

RFC 5782 notes that the name properly belongs to the MAPS RBL, the descendant of the original list, and that the conventional term is now DNSBL. In practice RBL, DNSBL and blocklist are used interchangeably. M3AAWG's sender practices, for example, mention proving consent to an RBL (Realtime Blackhole List) operator. Products often label their blocklist settings RBL, as Barracuda's Email Gateway Defense does with its custom RBLs page.