Lookup for "450 4.7.2"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (450)
Requested mail action not taken: mailbox unavailable [E.g., mailbox busy]
Enhanced Status Code Details (4.7.2)
Mailing list expansion prohibited
The sender is not authorized to send a message to the intended mailing list. This is useful only as a permanent error.