mx.sendinblue.com.mx has no CNAME records.
Question and response
Question
dig mx.sendinblue.com.mx. NS
Answer
The response has no answer records.
Authority
sendinblue.com.mx. 2830 SOA dns.sendinblue.com.mx. hostmaster.sendinblue.com.mx. 1887276444 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0