sd.cox.net has no MX records.
Question and response
Question
dig sd.cox.net. MX
Answer
The response has no answer records.
Authority
sd.cox.net. 86400 SOA dukensumpov1z.services.cox.net. hostmaster.cox.net. 781212461 28800 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0