Lookup for "251 5.5.2"
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 (251)
User not local; will forward to
Enhanced Status Code Details (5.5.2)
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.