Lookup for "553 2.1.7"
This status code indicates a successful delivery or other operation. Any details are simply notes.
Basic Status Code Details (553)
Requested action not taken: mailbox name not allowed [E.g., mailbox syntax incorrect]
Enhanced Status Code Details (2.1.7)
Bad sender's mailbox address syntax
The sender's address was syntactically invalid. This can apply to any field in the address.