Apex domains cannot have CNAME records.
Question and response
Question
dig send.hojmark.dk. NS
Answer
send.hojmark.dk. 5 NS ns1.klaviyo.com.
send.hojmark.dk. 5 NS ns4.klaviyo.com.
send.hojmark.dk. 5 NS ns3.klaviyo.com.
send.hojmark.dk. 5 NS ns2.klaviyo.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0