ns1.sendinblue.com has no CNAME records.
Question and response
Question
dig ns1.sendinblue.com. NS
Answer
The response has no answer records.
Authority
sendinblue.com. 1800 SOA josh.ns.cloudflare.com. dns.cloudflare.com. 2397916357 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0