Apex domains cannot have CNAME records.
Question and response
Question
dig cloudsmtp.com.br. NS
Answer
cloudsmtp.com.br. 5 NS ns-847.awsdns-41.net.
cloudsmtp.com.br. 5 NS ns-296.awsdns-37.com.
cloudsmtp.com.br. 5 NS ns-1583.awsdns-05.co.uk.
cloudsmtp.com.br. 5 NS ns-1503.awsdns-59.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0