Delayed Message: Message encryption issues
The email message has not yet been delivered because an email server involved with sending the message had difficulty processing the security features of the message.
Fixes
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.
4.7.6: Official Definition
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that start with 4.X.X:
A persistent transient failure is one in which the message as sent is valid, but persistence of some temporary condition has caused abandonment or delay of attempts to send the message. If this code accompanies a delivery failure report, sending in the future may be successful.
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that end with X.7.6:
A transport system otherwise authorized to validate or decrypt a message was unable to do so because the necessary algorithm was not supported.