Apex domains cannot have CNAME records.
Question and response
Question
dig sendmail.dk. NS
Answer
sendmail.dk. 5 NS ns2.dns-sec.dk.
sendmail.dk. 5 NS ns1.dns-sec.dk.
Authority
The response has no authority records.
Additional
ns1.dns-sec.dk. 5 A 92.241.203.106
ns2.dns-sec.dk. 5 A 92.241.203.107
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0