I've a question about the split domain setup as described in the wiki. If I set up zimbra.example.com to be the secondary server (so that mail first goes to the primary server, let's call it mail.example.com), then:

1) mail sent from the Internet to a user on mail.example.com goes directly to that user.
2) mail sent from a user on zimbra.example.com to a user on mail.example.com gets forwarded on as a wildcard unknown by zimbra.

But what about mail sent from the Internet (or a user on mail.example.com) to a user on zimbra.example.com? I'm running Postfix on my primary server, and I don't know what the equivalent might be to what the wiki suggests for Zimbra as a primary server, i.e., zmprov ma bar@example.com zimbraMailTransport smtp:mail.example.com. How do I tell Postfix to forward a particular virtual domain user to another machine for be handled there?

For example, I don't think simply aliasing bar@example.com to bar@zimbra.example.com will work; that's not how the user's known to the zimbra server, right? Or could I somehow create a mapping on the Zimbra system (forgive me for thinking aloud as I write) from @zimbra.example.com to @example.com, so that it handles such addressing?