mx.corelink.com has no CNAME records.
Question and response
Question
dig mx.corelink.com. NS
Answer
The response has no answer records.
Authority
corelink.com. 1616 SOA amber.ns.cloudflare.com. dns.cloudflare.com. 2397064424 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0