spf.sendinblue.com has no MX records.
Question and response
Question
dig spf.sendinblue.com. MX
Answer
The response has no answer records.
Authority
sendinblue.com. 1800 SOA josh.ns.cloudflare.com. dns.cloudflare.com. 2396909400 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0