Lookup for "501 2.2.2"
This status code indicates a successful delivery or other operation. Any details are simply notes.
Basic Status Code Details (501)
Syntax error in parameters or arguments
Enhanced Status Code Details (2.2.2)
Mailbox full
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.