fg.mymailboxes.fr has no CNAME records.
Question and response
Question
dig fg.mymailboxes.fr. NS
Answer
The response has no answer records.
Authority
mymailboxes.fr. 10800 SOA ns3.rmi.fr. ndd.adista.fr. 2023120101 21600 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0