Lookup for "554 5.6.0"
Permanent Error - This status code indicates a permanent issue and is unlikely to resolve itself. You should contact the Email Service Provider.
Basic Status Code Details (554)
Transaction failed
Enhanced Status Code Details (5.6.0)
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.