Encrypt email messages - Microsoft Support
https://support.microsoft.com/en-us/office/encrypt-email-messages-373339cb-bf1a-4509-b296-802a39d801dc
WEBIn an email message, choose Options, select Encrypt and pick Encrypt with S/MIME option from the drop down, You'll see an Encrypt with S/MIME if you have an S/MIME certificate installed on your computer. For Outlook 2019 and Outlook 2016, In an email message, choose Options, select Permissions.
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