On 18 Mar 2021, at 14:19, Paul Smith <paul(_at_)pscs(_dot_)co(_dot_)uk> wrote:
On 18/03/2021 13:51, John C Klensin wrote:
There are, I think, two unresolved question about the
interactions with extended reply codes. One is what a client is
expected to do it receives an unrecognized code --either one in
the ranges used by SMTP/5321 or completely out of range (666
anyone?)-- followed by some digits and periods that might be an
From our SMTP client's point of view. An *extended* status code can be
anything, but the basic status codes have to be 1xx to 5xx (xx can be any
non-space character - we're quite forgiving). A response of "451 8.5.1
whoopsie" would be treated as a response of 451 with text "8.2.1 whoopsie"
An invalid basic status such as '666' would be treated as a generic 5xx
response. I'm not really sure what else could be done with it.
We just use the first digit of the basic status code, and pretty much ignore
the extended status code. There's the *option* of doing something with it,
and the user (server administrator) can set it up to do something with it,
but, as far as I am aware, no one ever does, as it's too fragile, and gives
negligible benefit for our customers. (For mail servers handling more mail,
then I wouldn't be surprised if they do something with the extra digits, but
our software is aimed at SMEs, not at Gmail-wannabees.)
MTAs that send high volumes of email do pay attention to the other digits and
the extended codes. In fact, many of them do full text parsing to try and
determine how they should handle future mail to that address.
https://smtpfieldmanual.com is one example.
IME, the code details and extended codes are slightly useful for diagnostic
and troubleshooting, but, not really, as most of them are only useful to the
remote mail server "administrators", and in most cases those people haven't
got a clue what would trigger the different codes.
The code details and extended codes are very useful for high volume senders
trying to manage large bulk sends and their databases. They are parsed, and
tracked and updated so that senders can follow what is happening with their
sends. It is a challenge to programatically interpret the responses to manage
future sends to an address.
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