What about starting with Zimbra's Open Source edition, and building onto it?
Let me just step in here and say.. it's tough to build onto Zimbra. At work, we support ~1000 users on Zimbra (network edition), with hundreds of thousands of messages flowing through daily, and it doesn't like you tinkering with stuff under the hood. Most of your customizations get blown away when you upgrade. That said, I know of some organizations who customize it like crazy (I had heard that Lycos's free mail system is Zimbra-based, and Yahoo as well). Once you deviate, though, don't expect to stick to Zimbra's releases. It might be easier to just start fresh with postfix, amavis, spamassassin, dovecot, etc. We've also run into some pain in scaling it out (they want you to use Red Hat Clustering, but there's no great way to scale out the mail store regardless). Ryan