Apex domains cannot have CNAME records.
Question and response
Question
dig mailserver.comunicadoimportante.com.br. NS
Answer
mailserver.comunicadoimportante.com.br. 5 NS ns11.abovedomains.com.
mailserver.comunicadoimportante.com.br. 5 NS ns12.abovedomains.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0