Apex domains cannot have CNAME records.
Question and response
Question
dig serviciodecorreo.es. NS
Answer
serviciodecorreo.es. 5 NS ns-arsys.ui-dns.org.
serviciodecorreo.es. 5 NS ns-arsys.ui-dns.biz.
serviciodecorreo.es. 5 NS ns-arsys.ui-dns.es.
serviciodecorreo.es. 5 NS ns-arsys.ui-dns.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0