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From: Theo Van Dinter <felicity@kluge.net>
To: vernon@comp-wiz.com
Cc: spamassassin-talk <spamassassin-talk@example.sourceforge.net>
Subject: Re: [SAtalk] OT: DNS MX Record Clarification Please
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On Fri, Sep 13, 2002 at 01:33:43PM -0500, vernon wrote:
> As I understand it, DNS A records are used in a rotating fashion for load
> balancing, but DNS MX records are used in order or prority, meaning the 10
> before the 20 and only 20 if the 10 isn't available.

That's the theory, yes.

> But only some of the mail is actually being scanned which leads me to
> believe that not all of the mail is actually hitting that box and the 10
> never goes down. Why? Have I got something confused here?

No, but either due to some technical glitch, or downright just wanting
to do so, people send to the secondary.  It's a semi-usual spammer trick
actually to bypass the main server and send directly to a secondary
since it will either have less filtering, or be "trusted", or ...

MX records in the mail world are all explained in RFC 2821, section 5:

[...]
   Multiple MX records contain a preference indication that MUST be used
   in sorting (see below).  Lower numbers are more preferred than higher
   ones.  If there are multiple destinations with the same preference
   and there is no clear reason to favor one (e.g., by recognition of an
   easily-reached address), then the sender-SMTP MUST randomize them to
   spread the load across multiple mail exchangers for a specific
   organization.
[...]
   If it determines that it should relay the message without rewriting
   the address, it MUST sort the MX records to determine candidates for
   delivery.  The records are first ordered by preference, with the
   lowest-numbered records being most preferred.  The relay host MUST
   then inspect the list for any of the names or addresses by which it
   might be known in mail transactions.  If a matching record is found,
   all records at that preference level and higher-numbered ones MUST be
   discarded from consideration.  If there are no records left at that
   point, it is an error condition, and the message MUST be returned as
   undeliverable.  If records do remain, they SHOULD be tried, best
   preference first, as described above.

--=20
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