From: Ralf Hildebrandt (Ralf.Hildebrandtcharite.de)
Date: Mon Feb 06 2006 - 00:55:23 CST

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* Bob Tanner <tannerreal-time.com>:

> Have a very busy mailing list server running postfix, that frequently
> returns 4yz to sending MTAs.

Why?
 
> The way I understand the RFC, the sending MTA should (must?) retry my
> mailing list server "later" if a 4yz code is returned?
Yes.
 
> When postfix is the sending MTA it does the "right" thing. Later, I see the
> sending postfix retry my mailing list server.

Of course. That's why greylisting works.
 
> BUT sendmail, moves to the next mx host on a 4yz code.

Well, why not.

> Speaking with the sendmail MTA owner, I was told postfix implementation
> of RFC 2821, Section 5, paragraph 2 is "wrong".

That section has nothing to do with Postfix, since Postfix is the
receiving MTA in your case.

When the lookup succeeds, the mapping can result in a list of alternative
delivery addresses rather than a single address, because of multiple MX
records, multihoming, or both. To provide reliable mail transmission,
the SMTP client MUST be able to try (and retry) each of the relevant
    ^^^^^^^^^^^
    sendmail in this case
    
addresses in this list in order, until a delivery attempt succeeds.
However, there MAY also be a configurable limit on the number of
alternate addresses that can be tried. In any case, the SMTP client
SHOULD try at least two addresses.

And postfix does the same...

--
Ralf Hildebrandt (Ralf.Hildebrandtcharite.de) spamtrapcharite.de
Postfix - Einrichtung, Betrieb und Wartung Tel. +49 (0)30-450 570-155
http://www.postfix-buch.com
Microsoft is a cross between The Borg and the Ferengi. Unfortunately
they use Borg to do their marketing and Ferengi to do their
programming

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