Microsoft has confirmed a known issue that prevents recipients from opening encrypted emails in classic Outlook.

A new Microsoft-issued software patch for Outlook (Classic) on Windows 11 inadvertently broke encrypted email support - ...

There is no fix available, but you can always revert to an older version.

PCWorld reports that Microsoft has confirmed a bug in classic Outlook version 2511 preventing users from opening encrypted emails. This issue significantly impacts business users and organizations ...