Apex domains cannot have CNAME records.
Question and response
Question
dig emailemnuvem.com.br. NS
Answer
emailemnuvem.com.br. 4 NS ns1.skymail.net.br.
emailemnuvem.com.br. 4 NS ns3.skymail.net.br.
emailemnuvem.com.br. 4 NS ns2.skymail.net.br.
Authority
The response has no authority records.
Additional
ns1.skymail.net.br. 4 A 168.0.135.116
ns2.skymail.net.br. 4 A 168.0.134.16
ns3.skymail.net.br. 4 A 200.187.66.240
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0