Microsoft Outlook: Emails encrypted with S/MIME cannot be opened. The error message "Internal error." appears.

Assume the following scenario:

• A user receives an e-mail message encrypted with Secure/Multipurpose Internet Mail Extensions (S/MIME).
• The message cannot be opened.
• When opening the message, the following error message is displayed:

Unfortunately, there is a problem opening this item. This may be temporary. If this error occurs again, you should restart Outlook. Error in the underlying security system. Internal error.

The message can be opened via Outlook Web Access (OWA).

Occurs when both sender and recipient use Office 365. The sender uses Outlook for Macintosh and uses the Encrypt button.

Here, the message is not encrypted with S/MIME, but with an Office 365 proprietary method.

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If you inspect the message with MFCMAPIyou can see that the encrypted attachment is not named "smime.p7m", but "message_v2.rpmsg".

The sender must use the "S/MIME" button.

This is hidden by default in Outlook for Macintosh and must first be added to the toolbar.

After that, S/MIME encryption can be performed by clicking the button of the same name.

Related links:

External sources

• Learn about encrypted messages in Outlook.com (Microsoft)