Lookup for "551 4.7.7"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (551)
User not local; please try
Enhanced Status Code Details (4.7.7)
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.