mx3.altibox.net has no CNAME records.
Question and response
Question
dig mx3.altibox.net. NS
Answer
The response has no answer records.
Authority
altibox.net. 1719 SOA ns1.altibox.net. root.lyse.net. 2021073701 28800 7200 2419200 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0