Lookup for "553 4.3.0"
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 (4.3.0)
Other or undefined mail system status
The destination system exists and normally accepts mail, but something about the system has caused the generation of this DSN.