Spam Filtering for Mail Exchangers:
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# A.1. Prerequisites

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.

### Notes

[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:
    # apt-get install exim4-daemon-heavy

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