Microsoft Outlook: Signed e-mail messages appear invalid with error message "No certificate was found to verify the signature of this message."

Assume the following scenario

• A user receives an e-mail message signed with Secure/Multipurpose Internet Mail Extensions (S/MIME).
• The user (the recipient) uses Microsoft Outlook for Windows.
• The sender uses Microsoft Outlook for Macintosh.
• The certificate used to sign the message is valid.
• The e-mail signature is displayed as invalid. Inspection of the signature reveals that no details about the signature certificate can be displayed.

Error: No certificate was found to verify the signature of this message.
Signed by (certificate subject name unknown) using RSA/SHA256 at 15:44:59 19.05.2021.

Cause

The behavior seems to be a peculiarity of Outlook for Macintosh and apparently occurs when the option to send the signing certificate with the signed message is disabled.

Related links:

• Microsoft Outlook: Signed e-mail messages are rejected by the receiving mail server with error message "Invalid S/MIME encrypted message."