Apex domains cannot have CNAME records.
Question and response
Question
dig mailserver.servidoraweb.net. NS
Answer
mailserver.servidoraweb.net. 5 NS dns1.servidoraweb.net.
mailserver.servidoraweb.net. 5 NS dns2.servidoraweb.net.
Authority
The response has no authority records.
Additional
dns1.servidoraweb.net. 5 A 167.250.4.4
dns2.servidoraweb.net. 5 A 158.69.231.253
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0