Om Malik thinks Zimbra has done email right:
For starters, it does everything Outlook does, except in a browser. For instance, you can drag and drop emails and turn them into calendar events, or create appointments, and invite attendees. It senses phone-numbers, turns them into hyperlinks which can be clicked to initiate calls via Skype or SIP based systems. The company has just added added abilities to link directly to an Asterisk PBX system to make outbound calls from within the client.
It can also auto-find addresses, and using a Yahoo Maps (or Google Maps) Zimlet, give you directions to that location. It is ridiculously simple, that I wonder why more email clients dont have such support. Zimlets, are Zimbras version of plugins, or AppleScript, that can help you extend the clients functionality. I hope someone writes a great little zimlet that would let me blog emails to my WordPress blog.
I would go out on a limb and say that it combines the best of both Microsoft Outlook and Googles GMail! Plus, Zimbra has this search and save search feature that is very much like Apple Mails smart mailboxes.