mx.sisson-son.com has no CNAME records.
Question and response
Question
dig mx.sisson-son.com. NS
Answer
The response has no answer records.
Authority
sisson-son.com. 3559 SOA dns.sisson-son.com. hostmaster.sisson-son.com. 1548743523 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0