Brian Sniffen <bts@evenmere.org> writes:

> smtpmail.el treats all return codes over 400 as errors, and aborts.
> But RFC 821 and successors call the 400-series “transient” errors and
> suggest retrying immediately.  Some mail servers, including current MS
> Exchange, use 400-series errors for load limiting.  The user can just
> repeat C-c C-c until the mail goes through... or Emacs SMTPmail could
> loop until the server accepts it.
>
> Retrying is a little tricky in the case of multipart commands like
> LOGIN, but in general it’s safe to loop for seconds on any command.

This is what RFC 5321 says:

https://tools.ietf.org/html/rfc5321#section-4.2.1

4yz  Transient Negative Completion reply
      The command was not accepted, and the requested action did not
      occur.  However, the error condition is temporary, and the action
      may be requested again.  The sender should return to the beginning
      of the command sequence (if any).  It is difficult to assign a
      meaning to "transient" when two different sites (receiver- and
      sender-SMTP agents) must agree on the interpretation.  Each reply
      in this category might have a different time value, but the SMTP
      client SHOULD try again.  A rule of thumb to determine whether a
      reply fits into the 4yz or the 5yz category (see below) is that
      replies are 4yz if they can be successful if repeated without any
      change in command form or in properties of the sender or receiver
      (that is, the command is repeated identically and the receiver
      does not put up a new implementation).

So, indeed, smtpmail SHOULD try resending when given a 4xx response
code...  but presumably not forever, either.

It's a bit difficult to test, though.  Does the following patch work for
you?

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