Lookup for "553 4.2.0"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (553)
Requested action not taken: mailbox name not allowed [E.g., mailbox syntax incorrect]
Enhanced Status Code Details (4.2.0)
Other or undefined mailbox status
The mailbox exists, but something about the destination mailbox has caused the sending of this DSN.