Using sendmail to Block Spam:
The world will be grateful that your server is not a source for junk mail, but your users will be happy only if they are not the targets for spam. Two techniques that sendmail provides for blocking incoming spam are a DNS-based service to block spam sources, and a local database that controls access. This section examines both techniques.
Using the Realtime Blackhole List
The simplest way to block spam is to let someone else do it. sendmail allows you to use the
Realtime Blockhole List (RBL) that comes from the Mail Abuse Prevention System (MAPS). Visit the
website at mail-abuse.org/rbl to find out more about the MAPS system.
Using the RBL is very easy because the system is implemented through DNS. Every Linux system
can issue DNS queries, so this is a very effective way to distribute information. Of course, a
program can make use of the information only if it understands it. sendmail does. If you want to use
the MAPS RBL to block spam, add the following feature to your sendmail configuration:
FEATURE(`dnsbl')
With this feature enabled, mail from every site listed in the MAPS RBL is rejected.
There are many systems listed in the MAPS RBL. MAPS enforces a very stern policy. Any site that relays spam—which could be your site if you don't properly configure sendmail—is listed in the RBL. The RBL is one of the reasons why it is essential to configure relaying properly. A mistake in configuring relaying could get your site added to the blackhole list. If a site stops relaying spam, it is removed from the list after about a month. If your site gets added to the RBL, apply to have it removed from the list by following the instructions on the MAPS website.