mx.CenturyLink.net has no CNAME records.
Question and response
Question
dig mx.CenturyLink.net. NS
Answer
The response has no answer records.
Authority
centurylink.net. 153 SOA authns2.qwest.net. abuse.aup.lumen.com. 2260302000 300 300 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0