mx.centurylink.net has no NS records.
NS records for this zone are defined at centurylink.net.
NS records for this zone are defined at centurylink.net.
Question and response
Question
dig mx.centurylink.net. NS
Answer
The response has no answer records.
Authority
centurylink.net. 300 SOA authns2.qwest.net. abuse.aup.lumen.com. 2260120000 300 300 604800 300
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0