smtp.openip-cs.net has no CNAME records.
Question and response
Question
dig smtp.openip-cs.net. NS
Answer
The response has no answer records.
Authority
openip-cs.net. 9818 SOA dns1.openip.fr. root.openip.fr. 2026020401 10800 3600 604800 10800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0