For these examples, you need the Exim Mail Transport Agent, preferrably with Tom Kistner's
Exiscan-ACL patch applied. Prebuilt
Exim+Exiscan-ACL packages exist for the most
popular Linux distributions as well as FreeBSD; see the Exiscan-ACL
home page for details[14].
The final implementation example at the end incorporates these additional tools:
SpamAssassin - a popular spam filtering tool that analyzes mail content against a large and highly sophisticated set of heuristics.
greylistd - a simple greylisting solution written by yours truly, specifically with Exim in mind.
Other optional software is used in examples throughout.
[14]
In particular, Exim is perhaps most popular among users of
Debian GNU/Linux,
as it is the default MTA in that distribution. If you use
Debian (“Sarge” or later), you can obtain
Exim+Exiscan-ACL by installing the
exim4-daemon-heavy package:
# apt-get install exim4-daemon-heavy