mail.es.net has no CNAME records.
Question and response
Question
dig mail.es.net. NS
Answer
The response has no answer records.
Authority
es.net. 600 SOA ns1.es.net. hostmaster.es.net. 2026026431 3600 3600 1209600 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0