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 https://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 (https://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:
which means that all hosts are subject to RBL checking. If you wanted to
override blacklisting and accept mail from a specific host without
performing the RBL checking you could, for example, use:
rbl_hosts = ! nocheck.example.com : * |
The exclamation mark before the first item in this list indicates a
negated item: if the calling host is
nocheck.example.com,
it will match this item. But because of the negation, RBL checking is
not performed. Any other host matches the second item in the list.
