Lookup for "451 2.6.0"
This status code indicates a successful delivery or other operation. Any details are simply notes.
Basic Status Code Details (451)
Requested action aborted: local error in processing
Enhanced Status Code Details (2.6.0)
Other or undefined media error
Something about the content of a message caused it to be considered undeliverable and the problem cannot be well expressed with any of the other provided detail codes.