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[1].
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.
| [1] | 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:
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