Dear God no not Zimbra. Have you tried to deploy it in a corporate environment on terminal servers? Unless you can persuade the users to use the web ui (which nobody ever likes), you have to work in cached mode (zimbra client or Outlook connector). There is no online alternative. This caches the mailbox into a renamed PST. Worse, if you share other peoples mailboxes, it caches them as well. If you want a farm of terminal servers, you can try putting the cached ZDB files onto a network share (where they will break), or you can cache them for every user on every terminal server. Before long the amount of disk space you need more than outweighs the cost of the Exchange licensing! Zimbra just say there are no plans for an online mode. Until there is, it is a toy.