Lookup for "553 5.7.11"
Permanent Error - This status code indicates a permanent issue and is unlikely to resolve itself. You should contact the Email Service Provider.
Basic Status Code Details (553)
Requested action not taken: mailbox name not allowed [E.g., mailbox syntax incorrect]
Enhanced Status Code Details (5.7.11)
Encryption required for requested authentication mechanism
The selected authentication mechanism may only be used when the underlying SMTP connection is encrypted. This response code is documented for historical purposes only. Modern implementations SHOULD NOT advertise mechanisms that are not permitted due to lack of encryption, unless an encryption layer of sufficient strength is currently being employed.