ns.cox.net has no MX records.
Question and response
Question
dig ns.cox.net. MX
Answer
The response has no answer records.
Authority
cox.net. 3600 SOA dukensumpov1z.services.cox.net. hostmaster.cox.net. 781212129 28800 7200 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0