From Zimbra :: Wiki
Sharing Add-In Is Not Available
In some cases Outlook may disable the Zimbra Sharing Add-In. The Sharing Add-In is what enables you to share folders with others and access the contents other users have shared with you. If Outlook disabled the Sharing Add-In, you must manually enable it.
You don't see the sharing tab in the folder properties dialog
You don't see the 'Other Users Mailbox' in the 'File'->'Open' menu
Find the ZCO version
Navigate to Start -> Control Panel -> Add or Remove Programs, select "ZCS Connector for Outlook", and click the "Click here for support information" link. That will display the ZCO version.
Verify that the Sharing AddIn is not disabled
* Navigate to Help->About->Disabled Items * If Zimbra Sharing AddIn appears, select it and click the Enable button
- If you are using version 4.0.273 or newer, also
* Find the following registry key and set its value to 3 * HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\Outlook\Addins\SharingAddin.Addin\LoadBehavior
Verify the Zimbra Client Extension is enabled
- Navigate to Tools->Options->Other->Advanced Options->AddIn Manager
- Verify Zimbra Client Extension is Enabled
Extra Troubleshooting of Zimbra Client Extension
Note: On one install it became necessary to delete the outcmd.dat file before the registry key would stay changed. Otherwise it became necessary to run the registry key every single time outlook was closed. Info came from Sam in this post. Do not make the following mods unless you experience problems after changing the registry key as mentioned above.
- Close outlook.
- Open a command prompt and navigate to the %appdata%\microsoft\outlook folder. Delete the outcmd.dat file.
- Restore the LoadBehavior registry key to 3 and run outlook.
This still applies to the Windows Vistas users as well.