I've run Zimbra for 3 years now, back to 5.0.9, which I installed for my then employer. The architectural people there have taken, right along, an attitude that I can characterize only as "RFCs? Who cares about those?"
It doesn't handle fixed-pitch well; its editor won't re-wrap (though they might have finally fixed that in 7), it doesn't uknow from RFC 2369 -- in fact, it handles mailing lists poorly in general; notably, you can't change the Reply-To in any way when replying, if you generally want HTML off (as I do), the only way to turn it on is to dive into the Preferences and switch it, then reload; same turning off...
Check for bugs filed on their bugzilla by jra@baylink.com if you want a full list of the ignominy. But in general, I would say: evaluate it pretty thoroughly to see if you can deal with its crap before deploying.
It's *very* pretty. I just don't know if it's worth the trouble.