Lookup for "450 2.7.4"
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 (2.7.4)
Security features not supported
A message contained security features such as secure authentication that could not be supported on the delivery protocol. This is useful only as a permanent error.