DRBL stands for Distributed Realtime Black List. There was a MAPS RBL service
in the Net, that perfectly worked for a long time. Unfortunately, MAPS LLC
stops their public services by July 31, 2001, which in fact means the most
effective anti-spam engine in the Net is dead. 

DRBL differs from MAPS RBL in it's distributed nature. Instead of a
proprietary database controlled by certain people, DRBL offers every
network to establish its own database and share it with colleagues.
DRBL method of operation is similar to "old" RBL, but the main difference is
(1) using many "local" databases instead of one centralized and
(2) sharing information among them, so many other networks can make decisions
whether some server is a junk generator and has to be banned, and ever do that
automatically by getting and analyzing such information from different sources.

WWW: http://www.drbl.ofisp.org/eng/