I just looked it up through google "postfix transport table"
When no transport is specified, Postfix uses the transport that matches the address domain class. The following sends all mail for example.com and its subdomains to host gateway.example.com:
In the above example, the  suppress MX lookups. This prevents mail routing loops when your machine is primary MX host for example.com.
Although I'm not sure, I believe that this may allow you to not have to open up port 7025 to the zimbra server. Instead of :[gateway.example.com], you would put something like :[192.168.200.11] or whatever internal IP address you have associated with your zimbra mail server.