Mail spam, or unsolicited email advertising, is an annoying problem for many users. A project has been formed to address this problem called the Mail Abuse Protection System (MAPS), and a mechanism has been built that reduces the problem, called the Real Time Blackhole List (RBL). Information on how the MAPS RBL works can be obtained from its online documentation at http://maps.vix.com/rbl/. The idea is simple. Sites that are caught generating mail spam are added into the database and mail transfer agents like Exim are able to query the database to confirm that a source is not a spammer before accepting mail from it.
Since the advent of the RBL, several other similar lists have been created. One of the most useful is the Dial-Up List (DUL), which lists the IP addresses of dial-up hosts. These should normally send outgoing mail only to their ISP's mail servers. Many sites block mail from external dial-ups because when such a host avoids its own ISP's server, it is usually up to no good.
Exim provides support for the real-time and other blacklists. It is very easily configured. To enable it, add the following lines to your /etc/exim.conf file:
# Vixie / MAPS RBL (http://maps.vix.com/rbl) rbl_domains = rbl.maps.vix.com : dul.maps.vix.com
This example checks both the RBL and the DUL, rejecting any messages from hosts that are on either list. The rbl_hosts option allows you to specify groups of hosts to which RBL checking does (or does not) apply. The default setting is:
rbl_hosts = *
rbl_hosts = ! nocheck.example.com : *