mailx.es-core.net has no CNAME records.
Question and response
Question
dig mailx.es-core.net. NS
Answer
The response has no answer records.
Authority
es-core.net. 1800 SOA ns01.everscale-dns.com. postmaster.es-core.net. 2022040008 14400 604800 3600 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0