Apex domains cannot have CNAME records.
Question and response
Question
dig hb.d.sender-sib.com. NS
Answer
hb.d.sender-sib.com. 5 NS ns2.sendinblue.com.
hb.d.sender-sib.com. 5 NS ns1.sendinblue.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0