1. Zimbra is a small company.
2. Zimbra doesn’t integrate as tightly with Microsoft technology as Microsoft’s Exchange does.
3. Zimbra only offers Exchange-to-Zimbra migration.

That’s it folks. (Zimbra has recently been acquired by Yahoo so they aren’t that small any more)

Microsoft come up with some good reasons to go for Zimbra themselves such as:

• The open-source-software components [used by Zimbra] are proven solutions by themselves, many with large existing user bases (MySQL, OpenLDAP, Apache, PHP, SpamAssassin).
• Its’ [sic] class of service architecture allows for simple tiered access to services.
• Searching via their Web interface is fast and robust (AJAX query builder).
• The Verity HTML converter allows for fast rendering of 200 documents to HTML.
• Zimlets, can provide a more interactive user experience compared to the Microsoft equivalent of SmartTags.

Zimbra, on the other hand, have oodles of good stuff to say about Exchange:

• Exchange subscribes to proprietary interface to retain significant control over customer experience, while creating substantial reliance on Microsoft
• Microsoft’s agenda to progress its [Windows] business has resulted in lack of compelling support for Mac and Linux based products, non-Windows mobile devices and browsers, thereby limiting end-user choice in an otherwise diverse ecosystem
• Exchange 2007 is based on Microsoft Windows platform that is plagued with security issues that UNIX-based platforms are not.Arno Edelman, Microsoft’s European business security product manager, recently said “Microsoft is not a security company. Security is important, but it’s just a little part of Microsoft.” [I feel comforted.]

And there’s lots more too.

Nice one. If you are thinking about going down the Exchange or Sharepoint route, make sure you think very carefully…