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.
Seconded. In terms of functionality and interface, I like Zimbra a lot, but they make Microsoft and Apple look like amateurs in the "our way, or not all" game. As a small friends-and-family installation, I can't afford to dedicate a whole box exclusively to Zimbra[0], and trying to make it play nice with anything else running on the same server is a pain. As you say, pretty much anything that they don't have a GUI setting for is a nightmare to keep working across upgrades. I'd imagine it's actually better if you're planning a bigger-scale deployment and can have the architecture a lot more in line with how they expect it to be from the start. Regards, Tim. [0] OK, I probably could now with a VM, but the virtualisation support on my hosting box wasn't really there when I started...