fixed6.kpn.net has no MX records.
Question and response
Question
dig fixed6.kpn.net. MX
Answer
The response has no answer records.
Authority
fixed6.kpn.net. 3600 SOA ns7.kpn.net. zonemaster.kpn.com. 2020070102 900 600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0