Lookup for "451 4.4.6"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (451)
Requested action aborted: local error in processing
Enhanced Status Code Details (4.4.6)
Routing loop detected
A routing loop caused the message to be forwarded too many times, either because of incorrect routing tables or a user-forwarding loop. This is useful only as a persistent transient error.