as Zimbra Inc is a great company with great developers, and now with big bank credits (as Zimbra Inc has been acquired by Yahoo), I'd make some suggestions for the web UI.
Indeed, my users give me examples of what they need regularly :
As an admin, I'm happy for the Zimbra team that Yahoo acquired you, it'll allow you to develop more funny functionalities.
- message delivery acknowledgements so that they know when the recipient has opened the message
- a detail button in the To: field (when composing a mail) that permits to list the different email address that a distribution list contains
- a planning view that can show the availabilities of a company departement (for example, a chief can ask his staff to generate him a "simple" share than shows only availabilities), this view would be printable. This is useful for the reception hostess that need to know if users are available or not to answer to phone requests.
- print a popup when connecting to web UI while the "Out of office" message is activated (that permits to remember to deactivate this message when a user comes back from vacation).
- possibility to attach several files at a time
- have several appointments types (like in other products like Notes or Outlook), in order to differentiate several appointments into a calendar
- for the reception hostess, the need to create phone messages : someone let a message to them, so, they always need to create this type of message. This would ease their job (Domino has this functionality)
- add the 'i' shortcut in the shortcut list : this shortcut allows to refresh the screen.
- ability to just delete an attachment of a received email : sometimes, users receive some big files by email, so they need to detach them, but they need to keep the message in the mailbox. At the same time, they need to save some messages to RTF or DOC format in order to keep a paper printed version, but I've found a bugzilla vote for this function ...