Here is some info from my forum post. This is from the post on 8/24/11 at 8:31 AM
Zimbra is forcing offline after uninstall
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First, uninstall the current version normally. Then with regedit, go through HKLM and HKCU and search for the string "ZCS Connector for Outlook", and delete the corresponding left pane items. Sometimes, you'll want to delete the whole GUID, like with something like:
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Ins taller\UserData\S-1-5-18\Products. Here, the GUID would have a child key called InstallProperties, where the DisplayName value would be ZCS Connector for Outlook. In this case, you would want to delete the whole GUID in the left pane (the parent of InstallProperties).
You've got to be really careful doing this stuff, especially if you are squeamish. You should back up your registry before.
Note that I am not recommending this approach unless you are really confident in your abilities not to mess up your registry.
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Also, you'll want to delete HKCU\Software\Zimbra and HKLM\SOFTWARE\Zimbra. And delete any Zimbra dlls on your machine (in Program Files\Common Files\System\MSMAPI\1033 [anything starting with LSMS, SharingAddin or ShutdownAddin. Also a few .exe files like ZCOLogCtrl.