How to configure postfix to send messages received from gmail?

Question

gmail

There is an address [email protected] All emails coming to it are forwarded to the email in gmail.com, it is necessary that when answering, gmail.com sends letters as if from [email protected]
Previously, this could simply be configured in gmail settings -> Accounts and Import -> Add another email address you own and then select the option to verify with a code that the email really belongs to me. But now only the option "Send mail through your SMTP server" is available.
Now the server has postfix, which sends letters to the site, dkim + spf is configured. Now postfix is ​​covered by iptables.
How can I configure postfix in a strict way so that it would accept letters from the outside only from gmail (+ authentication) and local ones without authentication. All other attempts to send mail through my postfix were ignored.
It is desirable that postfix accept mail on port 587, I will open it in iptables.
Googled a lot, but the internet is full of articles on how to make postfix send emails via gmail. But I need the opposite.

Answer 1

For these purposes, the "return address" field is usually used.
Look here