Delayed Message: Unknown issue with the message content
The email message has not yet been delivered for unknown reasons related to its content or attachments.
If the message had attachments, try sending it again without them and then try sending each attachment in a separate email. If this error occurs when there are no attachments, check the text of the message for unusual content or characters.
If this error persists, contact the email provider for help.
4.6.0: 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 about the content of a message caused it to be considered undeliverable and the problem cannot be well expressed with any of the other provided detail codes.