A Realtime Blackhole List is a way to prevent SPAM by intentional loss of network connectivity.

The MAPS (Mail Abuse Prevention System) RBL works the following way:

  1. Everytime a Mail Transport Agent (MTA) ,such as sendmail, receives an E-Mail from a host, it checks if the host is listet in the RBL. If it is listed, the MTA rejects the E-Mail. The lookup is done via DNS. Example:
    An E-Mail comes from 192.5.5.1 . The MTA makes a lookup for 1.5.5.192.rbl.maps.vix.com. If the lookup is successful, your MTA knows that the host ist used by a spammer and will block the E-Mail.

  2. The MAPS has registered the AS 7777. This AS announces routes to the blocked hosts. You can configure your router to peer with this AS, but define something similar to /dev/null as the next hop, so all packets from or to spammers host will be thrown away.

    It's the hell for a postmaster if his mailserver is put on the RBL, because all of his users won't be able to send mail anymore because a lot of hosts block it.