RBL

from Jargon File (4.4.4, 14 Aug 2003)
RBL
 /R.B.L/

   Abbreviation: "Realtime Blackhole List". A service that allows people
   to blacklist sites for emitting {spam}, and makes the blacklist
   available in real time to electronic-mail transport programs that know
   how to use RBL so they can filter out mail from those sites. Drastic
   (and controversial) but effective. There is an RBL home page.