mail.greentelecom.net.br has no CNAME records.
Question and response
Question
dig mail.greentelecom.net.br. NS
Answer
The response has no answer records.
Authority
greentelecom.net.br. 9521 SOA ns344.hostgator.com.br. root.br344.hostgator.com.br. 2026010501 86400 7200 3600000 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0