Helo

Validate the HELO hostname. This plugin includes a suite of optional tests,
selectable by the policy setting. The policy section details which tests
are enforced by each policy option.

This plugin adds an X-HELO header with the HELO hostname to the message.

Using policy rfc will reject a very large portion of the spam from hosts
that have yet to get blacklisted.

WHY IT WORKS

The reverse DNS of the zombie PCs is out of the spam operators control. Their
only way to get past these tests is to limit themselves to hosts with matching
forward and reverse DNS, and then use the proper HELO hostname when spamming.
At present, this presents a very high hurdle.
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