Re: [ietf-smtp] SMTP Reply code 500

[Paul Smith <[e-mail]>]

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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