Lookup for "500 4.4.2"
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 (4.4.2)
Bad connection
The outbound connection was established, but was unable to complete the message transaction, either because of time-out, or inadequate connection quality. This is useful only as a persistent transient error.