How do I control whether Outlook performs native decryption of an SMime email?

Estois 0 Reputation points

I am currently working on an add-in which takes over the encryption and decryption process of emails with the IPM.Note.SMime Message Class from Outlook.

To enable this, I am currently toggling a registry key located at:

Path: HKCU\Software\Policies\Microsoft\Office\16.0\outlook\security

Key Name: externalsmime

Key Value: 1 (to force Outlook to handle externally)

Are there any other methods by which I can force Outlook to handle SMime message files externally other than this registry key?

Please let me know if more information is required, thanks in advance for the help!

This is for Outlook LTSC on Windows.

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Office Development
Office Development
Office: A suite of Microsoft productivity software that supports common business tasks, including word processing, email, presentations, and data management and analysis.Development: The process of researching, productizing, and refining new or existing technologies.
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  1. Nikita Mittal 0 Reputation points Microsoft Employee

    Hi Estois,

    We are working on adding support for Message Encryption for web add-ins:

    The development of the Outlook JavaScript API used by Outlook web add-ins is focused on closing the gap on scenarios that are only supported by VSTO and COM add-in solutions. This way, users who transition to the Outlook web add-in can continue to have a seamless experience.

    Let us know if there are any web add-in platform gaps preventing you from moving your COM add-ins to web at

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