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This article is about Kaspersky Endpoint Security for Windows (KES for Windows)

Outlook add-in failure may be related to a KES upgrade.

Step-by-step guide

As the first step to quickly fix majority of the issues with Outlook add-in, unregister it and register again. Here is how to do it properly:

1. Close Outlook if opened. 

2. Execute

   regsvr32 /u <product_folder>/mcou.dll regsvr32 /u <product_folder>/x64/mcou.dll

3. Delete all keys like HKLM\SOFTWARE\(Wow6432Node)\Microsoft\Office\Outlook\Addins\OutlookKLAvPlg.Addin*

4. Execute 

   regsvr32 <product_folder>/mcou.dll regsvr32 <product_folder>/x64/mcou.dll

5. Open MS Outlook, go to 

   File\ Options\ Add-ins\ field "Manage:", value "COM Add-ins", press button "Go..."\ window "COM Add-ins"

   Make sure the Kaspersky Outlook Anti-Virus Addin checkbox is selected.