how to determine who sent request to my smtp server: mail-client (such as outlook) or other smtp servers

There are several ways to know your SMTP server name. Below you find a list of the SMTP settings of the most common providers; if you are using a very local or ...

how to determine who sent request to my smtp server: mail-client (such as outlook) or other smtp servers

 am developing smtp server and there is a question i don't understand about smtp handshake.

#1. mail client (outlook) -> My SMTP Server EHLO - AUTH - FROM - TO - DATA - QUIT It is smtp relay request. My smtp server takes the request and delivers the mail to other mail server.

#2. Other SMTP Server -> My SMTP Server EHLO - FROM - TO - DATA - QUIT I understood smtp flow between SMTP servers as follows. AUTH command is not necessary in this case, because AUTH command is for authentication for client to send request relay to smtp server.

#1, #2 are the flow of SMTP that i understand. What I'm curious about is how does my smtp server determine whether this request is from a client or a server. I want to decide whether to do the AUTH command through it.

If my thoughts are wrong, don't laugh too much and i ask for a kind explanation. Thank you.


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