Apex domains cannot have CNAME records.
Question and response
Question
dig seppmaildemo.es. NS
Answer
seppmaildemo.es. 1 NS ns1.itmuller.com.
seppmaildemo.es. 1 NS ns2.itmuller.com.
seppmaildemo.es. 1 NS ns1.itmnetx.com.
seppmaildemo.es. 1 NS ns2.itmnetx.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0