Apex domains cannot have CNAME records.
Question and response
Question
dig mailgrid.net.br. NS
Answer
mailgrid.net.br. 4 NS ns-1608.awsdns-09.co.uk.
mailgrid.net.br. 4 NS ns-437.awsdns-54.com.
mailgrid.net.br. 4 NS ns-945.awsdns-54.net.
mailgrid.net.br. 4 NS ns-1289.awsdns-33.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0