Ah - so Google's IP will need to be added in your mynetworks as a relay or you can do something with 'relaydomains'. I don't really get the full complexity of your setup and why Google is in the mix. I thought you had a stand alone Postfix email server and was sending mail from a client to the server ... it appears that Google is the issue. I would check on the Postfix mailing list. Those guys can have you up and running in no time.