i'm sorry i wasn't clear, i was too tired yesterday

so i went here:
https://www.wormly.com/test_smtp_server
and tested my mail server with these:
sender email: user1@mydomain1.tld
recipient email: user2@mydomain2.tld
and the email sending succeed!
the "user1" part of the sender can be anything, the recipient must be a valid user.

i don't know which option in the config file permits this.
as i read in the postfix manual it is always a good idea to close smtpd_sender_restrictions with "reject", so now it looks like this:

smtpd_sender_restrictions = permit_mynetworks, permit_sasl_authenticated, reject

now it rejects the above emails.

EDIT:
the above leads to rejecting all incoming messages, i think i misread/misunderstand something, so FORGET THIS WHOLE TOPIC, i am sorry