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Zimbra, the web-based email client, has been working hard on a Zimbra Desktop tool, which is an offline, AJAX option for the Zimbra service. The latest version is now available for download, for Linux, Mac and Windows, though Windows users must re-sync.
The offline access for calendars is a new development that has been included in the most recent version of Zimbra Desktop. The batch message syncing tool has been improved, as well as any mailbox changes on the server side, as they're now reflected faster in the user interface. Overall, Zimbra Desktop mimics the web-based tools more, and switching from offline to online is more seamless. The search tools have also been improved, and the mail inbox will self-organize in a useful manner.