mx.coltfrance.com has no CNAME records.
Question and response
Question
dig mx.coltfrance.com. NS
Answer
The response has no answer records.
Authority
coltfrance.com. 600 SOA ns0.coltfrance.com. eudnsmaster.colt.net. 2021111700 21600 3600 604800 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0