Merle,
The incoming gateway server knows nothing of domains or whitelists/approved mail that resides on the primary mail server unless you manually add them (Domains tab within incoming gateway). It simply passes all mail unless you have specifically added any incoming spam config too. The only time the domains and config are shared is in a Failover configuration.
If you have specified domains it does try to confirm that the email address resides on the primary server, however if it cannot connect to the server it will hold the mail until such a time that the primary mail server comes back online for checking and delivery. That's how I see it anyway.