Lookup for "500 5.3.5"
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 (5.3.5)
System incorrectly configured
The system is not configured in a manner that will permit it to accept this message.