Lookup for "450 2.7.11"
This status code indicates a successful delivery or other operation. Any details are simply notes.
Basic Status Code Details (450)
Requested mail action not taken: mailbox unavailable [E.g., mailbox busy]
Enhanced Status Code Details (2.7.11)
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.