How to implement smtp and receive and process received emails?

Question

PHP

Good afternoon!
How to implement your own smtp protocol, provide user authorization by login and password and the ability to process commands?
An example of work: I authorize through any mail client and I need: "Ensure the execution of a list of commands with the issuance of a result to the client application using the standard network communication protocol"
when sending the help command, the server processes this and returns a list of commands.

Answer 1

Why do this at the application level?
You can use for this any MTA that knows how to external authorization, and the execution of external commands, when receiving messages. For example, some Postfix. And integrate your authorization and your message handlers to it.