Delayed Message: Unknown issue with mailbox
The email message has not yet been delivered because of a problem of some sort with the recipient's email inbox.
Check the recipient's email address for misspellings or missing letters. If an error is found, correct it in contacts list or address book for next time.
If necessary, contact the recipient another way (e.g., phone or text message) to confirm their email address.
4.2.0: 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.
The mailbox exists, but something about the destination mailbox has caused the sending of this DSN.