Delayed Message: Unknown issue
The email message could not be delivered because of an unknown issue.
Check the recipient's email for misspellings or missing letters. If you find an error, correct it in your contacts list or address book for next time.
If necessary, contact your recipient another way (e.g., phone or text message) to confirm their email address.
4.0.0: 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.
Other undefined status is the only undefined error code. It should be used for all errors for which only the class of the error is known.