mail.kpn.com.es has no CNAME records.
Question and response
Question
dig mail.kpn.com.es. NS
Answer
The response has no answer records.
Authority
kpn.com.es. 1681 SOA ns1.kpn.net. zonemaster.kpn.com. 2024041208 10800 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0