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Re: [ietf-smtp] SMTP Reply code 500

2020-03-04 03:39:52
On 04/03/2020 03:00, Geethapriya Liyanage wrote:
In SMTP communication, normally SMTP client sends commands and SMTP server replies to that with reply codes.  As in RFC 5321 , section 4.2.4, if the command is not recognized, code 500 should be returned. My doubt is, if the SMTP server sends unrecognized command or code to SMTP client , will the client also replies the same 500 code.  if not, how the SMTP client should handle it.

An SMTP server will never send a command to the SMTP client. That's just not SMTP. In SMTP, the client sends commands, the server sends responses, which all start with a three digit number where the first digit is 1 to 5

A buggy SMTP server could send an invalid response to the SMTP client  (ie a response not starting with a three digit number beginning with 1 to 5). In that case, the SMTP client should do whatever it wants to do, ranging from ignoring it, to dropping the connection in disgust and reporting an error to the user/administrator/log/whatever (I'd probably advocate for the latter).


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