Lookup for "500 4.6.1"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (500)
Syntax error, command unrecognized [This may include errors such as command line too long]
Enhanced Status Code Details (4.6.1)
Media not supported
The media of the message is not supported by either the delivery protocol or the next system in the forwarding path. This is useful only as a permanent error.