I like the kind of projects like Zimbra. I was active in the past in other webmail communities such as Uebimiau (Not the same audience, I know!), but I found Zimbra quite a heavy and complicated project. I think using JSP is overkilled, for an enterprise application I would have used maybe Python, but JSP is heavy for the server, and requires 5x more code
I read the documentation (PDF) of Zimbra, and it seems that we'll need a steep learning curve to play around with the API, framework, etc...
The source code is really a lot. I think you could have built a very nice web application based aggressively on AJAX + Python/PHP with less code. But in the end, Zimbra is quite revolutionary, it will attract at least a lot of attention.
Just my opinion