Postfix advanced relaying config/control

Question

How do I make Postfix only accept outbound mail from $mynetworks if the from is *@mydomain(s) ?

To be clear (and differentiate from similar questions I found) I want to make postfix such that users in $mynetworks, not subject to usual relay limitations, cannot send mail out using this smtpd for domains it does not control (ie: prevent sending out mail from gmail/hotmail etc using the internal postfix server), but still be able to send

Thanks! -SM

Edit: Let me re-simplify the question. I want to restrict relaying for $mynetworks so that mail is only accepted if it is from $relay_domains - an inverse of sorts of the default relay restriction for outside hosts, where mail is only accepted if the mail is to $relay_domains

Answer 1

In case anyone else is ever required by organizational policy to do this, I've found out how to enforce this restriction:

smtpd_sender_restrictions=reject_unknown_sender_domain.

Thanks to everyone for looking anyway.