Apex domains cannot have CNAME records.
Question and response
Question
dig mav.com.br. NS
Answer
mav.com.br. 5 NS dns5.m9.network.
mav.com.br. 5 NS dns6.m9.network.
mav.com.br. 5 NS ns1.supramail.com.br.
mav.com.br. 5 NS ns2.supramail.com.br.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0