The MAPS (Mail Abuse Protection System) RBL (Realtime Blackhole List) is a means of identifying hosts that have been associated with the sending of spam mail. A full description of the service and the technology and ethics behind it can be found at http://maps.vix.com/rbl/ along with more general mail policy information at http://maps.vix.com/.
Exim can use the MAPS RBL and/or any other similarly defined service (ie you could make your own additional maps as well). To use exim for this you need to be running version 1.80 or later.
This documentation is for versions of exim before version 3.00, which are now considered obsolete. The current information can be found in the most recent version of this howto.
These are fully detailed in the Exim Specification Document. The specific section on RBL is here and the rbl directives are documented starting here
The 2 standard RBL configurations are:-
rbl_domains = rbl.maps.vix.com no_rbl_reject_recipients rbl_warn_header
rbl_domains = rbl.maps.vix.com rbl_reject_recipients
In both cases machines can be explicitly removed from RBL control by adding their network addresses to the rbl_except_nets list.
The information to do more complicated manipulations can be found in the specification document and is outside the scope of this note.