As of March, 2010, we are running Zimbra Network Professional Edition version 6.0.5.  In most cases 01.com tests both the release and its associated software such as Outlook and iSync connectors prior to installing the latest releases.

This "sand-boxing" typically means 01.com is about 30 days behind the release of the most recent version of Zimbra in general release.  Our sandbox is a Zimbra multi-node installation that simulates our production Zimbra cloud hosting environment.  While we recognize a primary value of Zimbra is its rapid pace of development, we are conservative about our upgrades, prioritizing the stability and reliability of the 01.com Zimbra cloud over new features and bug-fixes.  See our testing procedures.

If you'd like to read more about Zimbra 6, and see some screen shots from it, please see our screen shots section here in our public FAQs, or visit our blog.

Based on our experience upgrading from version 3 to 4, and 4 to 4.5, and 4.5 to 5, we have scheduled the upgrade to Zimbra 6 for March 6, 2010. 

You may be interested in our next Scheduled Maintenance, check our public status and reporting page.

Also, you may want to see the Zimbra Roadmap, with features listed about Zimbra 6.