After piddling around with this for a complete day I have finally figured out what steps need to take place in order to do this correctly.
The first step is to do the Split Domain instructions for Zimbra as a secondary server. Then on your primary server, the OS X server, do the following.
In /etc/postfix/main.cs add this line if it doesn't already exist
Then in the transport file (/etc/postfix/transport) add the following lines for each user you want to have on the Zimbra server:
transport_maps = hash:/etc/postfix/transport
Now execute the following command:
firstname.lastname@example.org smtp: zimbra.example.com
And finally, restart postfix: