Apple Server - how to exclude a subdomain?
I'm starting this answer to talk through possibilities and will edit along the way:

As of Snow Leopard 10.6 apples MTA is Postfix you can find their docs here:
http://manuals.info.apple.com/en_US/MailServicesAdmin_v10.6.pdf

http://www.postfix.org/documentation.html

1) On the OS X server, at a terminal, dig mx test.example.com  What are the results? If it's the apple server, you need to change DNS records to point to gmail.  (possibly on this same os x server, as it may see itself as authoritative for this domain)