Message Not Delivered: Mailbox is full
The message could not be delivered because the recipient's email inbox is full.
Fixes
Contact the recipient another way (e.g., phone or text message) and ask them if they're aware that their mail box is full
The recipient can delete messages to make more space available
5.2.2: Official Definition
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that start with 5.X.X:
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.
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that end with X.2.2:
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.