mx6.centerasecurity.dk has no CNAME records.
Question and response
Question
dig mx6.centerasecurity.dk. NS
Answer
The response has no answer records.
Authority
centerasecurity.dk. 1658 SOA curt.ns.cloudflare.com. dns.cloudflare.com. 2397240896 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0