Apex domains cannot have CNAME records.
Question and response
Question
dig mailcorp.com.br. NS
Answer
mailcorp.com.br. 4 NS www01.ctinet.com.br.
mailcorp.com.br. 4 NS gw01.ctinet.com.br.
mailcorp.com.br. 4 NS dns1.ctitech.net.br.
mailcorp.com.br. 4 NS dns2.ctitech.net.br.
Authority
The response has no authority records.
Additional
dns1.ctitech.net.br. 4 A 177.190.207.234
dns2.ctitech.net.br. 4 A 186.193.234.22
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0