Apex domains cannot have CNAME records.
Question and response
Question
dig mailcloud.com.br. NS
Answer
mailcloud.com.br. 5 NS skip.ns.cloudflare.com.
mailcloud.com.br. 5 NS kim.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0