Lookup for "450 4.5.0"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (450)
Requested mail action not taken: mailbox unavailable [E.g., mailbox busy]
Enhanced Status Code Details (4.5.0)
Other or undefined protocol status
Something was wrong with the protocol necessary to deliver the message to the next hop and the problem cannot be well expressed with any of the other provided detail codes.