Hi,
Outlook 2kx support requires Zimbra Network Edition, and so does the BES environment, and I think you are currently using the Open Source version.
The open source version is freely available for download, while the Network edition is available under a yearly subscription fee per account.
In exchange you will get techsupport, and a number of additional features, such as support for mobile devices like the blackberry, and outlook.
sales@zimbra.com will probably be able to tell you more about this.
Good luck with your search !