postfix gateway reject incoming spoofed emails from my domain

I have a postfix gateway. It only allows emails from the outside to a list of inteal users in mydomain.com and it allow emails from my inteal mail server to go out to the inteet. There is no authentication happening on the gateway.

I would like a simple way to reject any incoming emails with a from address in mydomain.com while still allowing outgoing emails from my inteal mail server (preferably only ones with a from address of mydomain.com).

I already have SPF enabled but I can't make it too strict for various reasons unrelated this issue.