mail.ctbctelecom.com.br has no CNAME records.
Question and response
Question
dig mail.ctbctelecom.com.br. NS
Answer
The response has no answer records.
Authority
ctbctelecom.com.br. 1572 SOA tricia.ns.cloudflare.com. dns.cloudflare.com. 2392812011 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0