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[ietf-smtp] Semantics of 554 greeting? (Clarify in 5321bis?)

2019-12-17 14:07:38
When describing why Postfix defers most policy controls to "RCPT TO",
I wrote:

On Dec 16, 2019, at 12:45 AM, Viktor Dukhovni 
<ietf-dane(_at_)dukhovni(_dot_)org> wrote:

When an SMTP server returns 5XX in either the greeting or in response to
EHLO, some clients treat that as a connection setup failure, not a
permanent message delivery failure, and move on to the next MX host, and
perhaps ultimately defer and retry the message:

   smtp_skip_5xx_greeting (default: yes)

       Skip remote SMTP servers that greet with a 5XX status code.

       By default, the Postfix SMTP client moves on the next mail
       exchanger. Specify "smtp_skip_5xx_greeting = no" if Postfix
       should bounce the mail immediately.

The reason for that was that at least at the time, various Microsoft
SMTP servers woul return 5XX greetings under transient high load.
Consequently, Postfix itself is one of the clients that treats
5XX as temporary failures in greetings (though not in EHLO).

Which brings me to the question of this new thread, what is the
meaning of a 554 greeting with respect to the sender's envelope?

* Is a 554 greeting semantically equivalent to TCP RST?  With the
  client moving on to the next MX host or deferring the message?

* Or, is the client expected to conclude that the recipient domain
  does not accept email and bounce all the envelope recipients?

The text is unclear.  Postfix assumes the former, and perhaps that's
actually the intent of the RFC?  Rereading this text recently, I am
not sure what it means.


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