Lookup for "553 2.7.9"
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 (2.7.9)
Authentication mechanism is too weak
The selected authentication mechanism is weaker than the server policy permits. Client should retry with a new authentication mechanism.