Delayed Message: Unknown security or policy concern
The email message has not yet been delivered because the message has caused a security concern, but it is not clear what caused the problem.
Retry the email message see if it was a one-time problem or if the same error occurs.
Otherwise, contact the sender's email support team as the email system may have a security issue.
4.7.0: Official Definition
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.
Something related to security caused the message to be returned, and the problem cannot be well expressed with any of the other provided detail codes. This status code may also be used when the condition cannot be further described because of security policies in force.