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

[Laura Atkins <[e-mail]>]

> On 18 Mar 2021, at 14:19, Paul Smith <[e-mail]> 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. 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. 

laura 

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