Lookup for "504 5.7.7"
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 (504)
Command parameter not implemented
Enhanced Status Code Details (5.7.7)
Message integrity failure
A transport system otherwise authorized to validate a message was unable to do so because the message was corrupted or altered. This may be useful as a permanent, transient persistent, or successful delivery code.