Delayed Message: Mailbox is full
The email message has not yet been delivered because the recipient's email inbox is full.
The sender's email system will probably retry this message automatically and send updates.
However, you may want to contact the recipient another way (e.g., phone or text message) and ask them if they're aware that their mail box is full
4.2.2: 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 is full because the user has exceeded a per-mailbox administrative quota or physical capacity. The general semantics implies that the recipient can delete messages to make more space available. This code should be used as a persistent transient failure.