Delayed Message: Mailbox is not accepting new messages
The email message has not yet been delivered because the recipient's email inbox is currently not accepting new messages.
Contact the recipient another way (e.g., phone or text message) to see when their email will be running again.
Alternatively, try sending the message again in a few hours, or tomorrow.
4.2.1: 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 is not accepting messages. This may be a permanent error if the mailbox will never be re-enabled or a transient error if the mailbox is only temporarily disabled.