The problem happens for users who encrypt the email using the settings under the email File dialog (File > Encrypt), Microsoft further explained. To prevent the issue, there are two possible ...
A new Microsoft-issued software patch for Outlook (Classic) on Windows 11 inadvertently broke encrypted email support - ...
Classic Outlook tells you to open the email to "read its contents and verify your credentials," but that doesn't work.
Microsoft has confirmed a known issue that prevents recipients from opening encrypted emails in classic Outlook.
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