Lookup for "251 4.2.2"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (251)
User not local; will forward to
Enhanced Status Code Details (4.2.2)
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.