There doesn't seem to be a forum for Zimbra Desktop feature suggestions, so I'm posting here.
1. If synchronize is set to manual, every mailbox must be synchronized individually. A Synchronize All button would be helpful. Along with that, bold the accounts with unread mail.
2. A copy/move feature between mail accounts would be very useful.
3. Messages that are being composed can easily be lost. When closing them, it would make sense to ask if you want the message to be saved. Also, regularly auto-saving to drafts can be a lifesaver in case of crashes. That's one of my favorite Outlook behaviors.