I'm planning a migration from our existing SMTP (exim) and IMAP (cyrus) infrastructure to Zimbra Network Edition, now that our PO is signed for it...

What I'd like to do, if possible, is to have the new Zimbra system pass all of its messages through our existing SMTP server, and then migrate accounts a few users at a time with address rewriting and aliases. For example, bob@domain.com has an account on the Zimbra server, but needs to send email to alice@domain.com (same domain that Zimbra thinks it owns) that hasn't been converted yet. For that to happen, all email from Zimbra needs to be routed through the exim server, then exim will determine the destination from aliases and redirect bob's email to bob@zimbrahost.domain.com.

Sooo, the question becomes, do all Zimbra's messages pass through Zimbra's postfix instance, and if so, what do I need to change in main.cf in order to force it to route all smtp traffic through eximhost's SMTP?