Apex domains cannot have CNAME records.
Question and response
Question
dig mailtec.com.br. NS
Answer
mailtec.com.br. 5 NS ns-879.awsdns-45.net.
mailtec.com.br. 5 NS ns-215.awsdns-26.com.
mailtec.com.br. 5 NS ns-1969.awsdns-54.co.uk.
mailtec.com.br. 5 NS ns-1428.awsdns-50.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0