Re: [exim] Extremely Simple(?) greylisting with exim?

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Author: Kjetil Torgrim Homme
To: Chad Leigh
CC: exim users
Subject: Re: [exim] Extremely Simple(?) greylisting with exim?
On Tue, 2006-12-26 at 12:29 -0700, Chad Leigh -- Shire.Net LLC wrote:
> I set up two low priority MX hosts that are virtual hosts
> on the same machine as the normal mailhost. These lower priority
> ones automatically defer everything that hits them. [...]
> I do greylisting use Marc Merlin's sa-exim with his greylisting
> module added.

this will break when the sending server uses Exim...

here's the scenario:

      * server sends message to A (highest MX priority), and is defered
        due to greylisting.
      * server now tries to send message to B (lowest MX priority) which
        always defers.
      * server notes that B defers in retry database
      * some time later, it retries A, and it is successful.

repeat this for some time. eventually you get this:

      * server sends message to A, and is deferred due to greylisting.
      * server tries B, which defers.  it looks in retry database, sees
        that B hasn't worked for a long time, and BOUNCES THE MESSAGE

there are two ways to solve this:

     1. A should whitelist all servers which make successful deliveries,
        and *never* clean the whitelist.  as soon as even *one* message
        is deferred by A, the retry rule for B may hit and cause a
     2. don't use greylisting and "always deferring MX" together.
        either is fine, but combining them causes weird behaviour.
        there isn't really any point to it, anyway.

Kjetil T.