Lookup for "551 5.5.4"
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 (551)
User not local; please try
Enhanced Status Code Details (5.5.4)
Invalid command arguments
A valid mail transaction protocol command was issued with invalid arguments, either because the arguments were out of range or represented unrecognized features. This is useful only as a permanent error.