Lookup for "551 4.5.4"
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 (4.5.4)
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.