mx6.kiubi.com has no CNAME records.
Question and response
Question
dig mx6.kiubi.com. NS
Answer
The response has no answer records.
Authority
kiubi.com. 197 SOA ns1.amen.fr. root.amen.fr. 2026020402 86400 7200 2592000 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0