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Re: After a 450, queue or try next MX?

2006-08-30 12:07:40


Tony Finch wrote:
On Wed, 30 Aug 2006, Alex van den Bogaerdt wrote:
Where does it say the client should stop trying after a 5xx message?
And I mean stop trying the entire list, not just that one host?

4.2.1 Reply Code Severities and Theory

   5yz   Permanent Negative Completion reply
      The command was not accepted and the requested action did not
      occur.  The SMTP client is discouraged from repeating the exact
      request (in the same sequence).

i.e. the same seqence of commands, regardless of destination host.

"regardless of destination host" might be a reasonable decision, but I certainly do not see it implied by the language of the specification.

More importantly, this thread demonstrates that there are a variety of reasonable -- if conflicting -- choices to be made. So, the specifications are not going to impart divine wisdom.

At that point, the discussion ought to be about what is the most productive choice for the originator and the recipient.

On reflection, my own opinion is that the model of using an MX when a connection fails makes sense. If an SMTP connection is established, then a server has stepped forward to participate. The client should pursue matters with that server, unless and until that server drops out. A 4xx response does not constitute dropping out.

Hence, a 4xx response strikes me as warranting a later try, either directly with that same host, or with a fresh DNS query.



  Dave Crocker
  Brandenburg InternetWorking