smtp.celestemail.com has no CNAME records.
Question and response
Question
dig smtp.celestemail.com. NS
Answer
The response has no answer records.
Authority
celestemail.com. 3510 SOA ns1.celeste.fr.celestemail.com. support.celeste.fr. 2024100301 3600 900 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0