mx.corelink.com has no NS records.
NS records for this zone are defined at corelink.com.
NS records for this zone are defined at corelink.com.
Question and response
Question
dig mx.corelink.com. NS
Answer
The response has no answer records.
Authority
corelink.com. 1800 SOA amber.ns.cloudflare.com. dns.cloudflare.com. 2397064424 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0