SMTP: In which order which "from" email address would be used?

Question

I'm currently coding from scratch a new SMTP server and I'm asking in which order "from" address counts?

If I receive an email with the following content:

Return-Path: foo1@bar.tld
...
From: Foo Bar <foo2@bar.tld>

and the email was received by the SMTP server with the command:

MAIL FROM: foo3@bar.tld

And my server wants now to answer on this email, on which address should it answer? I guess the order would be first of the email header the "From:" address, then "Return-Path:", and if all this would be missing, then the address of the "MAIL FROM" command of the SMTP command?

Answer 1

Return-Path is used for sending bounce messages (i.e. delivery status notifications), replies should be sent to Reply-to if it exists, otherwise From.

Note that when the mail is received with

MAIL FROM: <foo3@bar.tld>

the Return-Path header should be updated to contain that address.