If you're looking for a speedy and stable dns server (authoritative or recursive), I'd recommend Dan Bernstein's dnscache and tinydns programs. I haven't used BIND since 2000, and will never go back.
http://cr.yp.to/djbdns.html
(This is the same guy that gives us qmail.) You can seperate locations "public and private" based on a defined access list, etc. It's industrial-strength. I worked for one of the top web sites in the world, and they use djbdns exclusively (do Windows domain controllers count?).
As for networking with Zimbra, we use zimbra behind a firewall on a non-routable IPv4 subnet, and use apache as an SSL proxy for external access, where we can restrict access to only the appropriate zimbra URLs.
--Brian