Lookup for "500 5.7.2"
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 (500)
Syntax error, command unrecognized [This may include errors such as command line too long]
Enhanced Status Code Details (5.7.2)
Mailing list expansion prohibited
The sender is not authorized to send a message to the intended mailing list. This is useful only as a permanent error.