Here's what I did to make postfix use mx records instead of local for a specific domain...
Edit the following file
Update your transport maps if it's not set.
I added hash:/etc/postfix/transport
transport_maps = hash:/etc/postfix/transport, proxy:mysql:/etc/postfix/mysql-virtual_transports.cf
made the MX changes to domain for google apps
make the following addition in your /etc/postfix/transport file.
It'll force postfix to look at your MX records rather than trying to deliver the mail internally.
Run the following command to hash your transport file.
Reload your postfix configuration