novaflash wrotePostfix has a concept called trusted networks. Trusted networks by default do not need to authenticate and can send emails without authentication. So if your add your VPN client network to the trusted networks, in theory it should work. But this is really more of a postfix question than Access Server. Good luck.
I really appreciate for your help. Do you know how to add my VPN home network to the trusted network by command?
I already added it to postfix side.