Lookup for "551 4.5.2"
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 (4.5.2)
Syntax error
A mail transaction protocol command was issued which could not be interpreted, either because the syntax was wrong or the command is unrecognized. This is useful only as a permanent error.