Apex domains cannot have CNAME records.
Question and response
Question
dig sendinblue.com.mx. NS
Answer
sendinblue.com.mx. 5 NS ns1.sendinblue.com.mx.
sendinblue.com.mx. 5 NS ns2.sendinblue.com.mx.
Authority
The response has no authority records.
Additional
ns1.sendinblue.com.mx. 5 A 92.42.201.193
ns2.sendinblue.com.mx. 5 A 5.135.200.50
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0