Message Not Delivered: Message encryption issues
The message could not be delivered because an email server involved with sending the message had difficulty processing the security features of the message.
If intentionally sending ecrypted email messages, check the sender's email program settings and make necessary changes. Retry the email message see if the same error occurs, or if it was a one time problem.
Otherwise, contact the sender's email support team or encryption provider to help identify any misconfiguration with the sender's account or encryption tools.
5.7.6: Official Definition
A permanent failure is one which is not likely to be resolved by resending the message in the current form. Some change to the message or the destination must be made for successful delivery.
A transport system otherwise authorized to validate or decrypt a message was unable to do so because the necessary algorithm was not supported.