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Updated April 29, 2009 at 8:45 am with more details about integrating third-party services.

We've been keeping our eye on Zimbra Desktop, the e-mail client that Yahoo acquired in 2007 and held onto for about a year before development work picked up again in earnest. Now, more than a year later, Zimbra Desktop 1.0 has shaken off its beta and is available as a free download for Windows and Mac.

Zimbra differentiates itself from Mozilla's Thunderbird e-mail client (Windows|Mac) and from Gmail in its amphibian nature as both an online and offline in-box. It also sees itself as a central in-box for all your e-mail, contacts, and calendar information. As such, you're able to access Yahoo and Gmail contacts, calendars, and messages in Zimbra, plus POP or IMAP e-mail from AOL, Hotmail, or your office. At least, that's in theory. According to Zimbra's Web site, syncing to some of the third-party e-mail and calendar services within Zimbra Desktop appear to remain beta features, like Hotmail, and Yahoo and Google calendars. While the support for these last two has been expanded in this release, a Zimbra representative told us, "officially those integrations are still considered 'beta' since they rely on APIs not maintained by the Zimbra team."

Unlimited storage and support for 20 languages rounds out the feature overview.

A slew of bug fixes and back-end tune-ups update the most recent beta version of Zimbra to its 1.0 release, a representative from Zimbra told CNET. Plus, there is now greater diversity in sharing Zimbra documents, and full support for Yahoo and Google contacts and calendars, in addition to Web mail.

While Zimbra Desktop 1.0 is free for personal use, Zimbra has been making Yahoo money through Zimbra Collaboration Suite, a hosted e-mail solution for schools and enterprise businesses like RedHat and 21st Century Realty Group.

Jessica Dolcourt reviews smartphones and cell phones, covers handset news, and pens the monthly column Smartphones Unlocked. A senior editor, she started at CNET in 2006 and spent four years reviewing mobile and desktop software before taking on devices.