DuckCorp Dico

Found one definition

1.                 From The Jargon File (version 4.4.7, 29 Dec 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.