[Exim] exim + smtp reverse dns

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Hey, I'm wondering how I could get Exim4 to, by default, not allow
non-reversable DNS entries, and also no MX entries in DNS, for all
SMTP<->SMTP traffic, except authenticated users.

After weeks of looking at why spam is marked as spam, I've pretty much figured
that pretty much all that spam comes from non-reversable IP addresses.

I'm pretty much using the default exim 4.0x ACL ruleset, but I'd rather have a
good understanding of how to implement this additional rule, before testing
it out live with possible failures.

Eric Renfro
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[ my pgp key of type application/pgp-keys deleted ]
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