smtp.flexnetwork.fr has no CNAME records.
Question and response
Question
dig smtp.flexnetwork.fr. NS
Answer
The response has no answer records.
Authority
flexnetwork.fr. 10175 SOA ns1.flexnetwork.fr. noc.flexnetwork.fr. 2025041402 10800 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0