We're testing out Zimbra and trying hard to break it. :-)
One thing I've run across is this: When I delegate my Inbox to another user and then I try to have that user take one of my messages and file it in a subfolder under 'Inbox' it comes back with an error:
msg: cannot copy object: -290
data: [object Object]
I did a quick search of the bug database and I may have missed it. If it's not a bug perhaps there is something that I'm not doing right.