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Re: [ietf-smtp] [ dispatch] Forced SMTP redirects

2021-03-18 09:55:42

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
extended code?

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. 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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