Apex domains cannot have CNAME records.
Question and response
Question
dig mailgrid.com.br. NS
Answer
mailgrid.com.br. 5 NS ns-177.awsdns-22.com.
mailgrid.com.br. 5 NS ns-889.awsdns-47.net.
mailgrid.com.br. 5 NS ns-1325.awsdns-37.org.
mailgrid.com.br. 5 NS ns-1838.awsdns-37.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0