Message Not Delivered: Mailbox is not accepting new messages
The message could not be 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.
5.2.1: Official Definition
A permanent failure is one which is not likely to be resolved by resending the message in the current form. Some change to the message or the destination must be made for successful delivery.
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.