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

Assume the following scenario:

• A user sends an e-mail message signed with Secure/Multipurpose Internet Mail Extensions (S/MIME).
• The sender uses Microsoft Outlook for Macintosh.
• The receiving mail server rejects the message and sends back a Non-Delivery Report (NDR):

550 5.6.0 M2MCVT.StorageError.Exception: ConversionFailedException - , Content conversion: Invalid S/MIME encrypted message.; storage error in content conversion.

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Occurs if GNUPG is implemented on the receiving side and the "Send digitally signed messages as clear text" option is not enabled on the sender side.

The documentation for this setting states:

Select this option if you want the contents of the message to be readable for all recipients. This includes recipients without an S/MIME mail application. A recipient without an S/MIME mail application can read a clear text message but can't verify the digital signature.

Related links:

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

External sources

• Digital signing and encryption settings (Microsoft)
• Content conversion: Invalid S/MIME encrypted message. (GNUPG Project)