Postfix must forward mail that it does not have in the configuration to another server

Hi everyone, I'm moving from an integrated Domino configuration with Office365 to a postfix e Office 365.
I have a problem that I can't overcome, I hope you can help me!

Situation

on an internal Ubuntu 18.04 server I configured postfix with some mailboxes

POSTFIX_SERVER
mymail1@mydomain.com
mymail2@mydomain.com
mymail3@mydomain.com

While other mailboxes are configured on an external mail server provider

REMOTE_SERVER
mymail4@mydomain.com
mymail5@mydomain.com
mymail6@mydomain.com

My problem is this:
I send an email to the server POSTFIX_SERVER, he must verify that the box exists in his configuration and accept it, otherwise he must forward it to my REMOTE_SERVER (Office365) (and then he will manage it with a delivery or an error)

I tried to see the Smarthort and Relayserver option, as I'm using on a domino server, but the functionality is different

Thankyou !