mx-corp.sercomtel.com.br has no CNAME records.
Question and response
Question
dig mx-corp.sercomtel.com.br. NS
Answer
The response has no answer records.
Authority
sercomtel.com.br. 1531 SOA ns1.skymail.net.br. hostmaster.sercomtel.com.br. 2026021803 7200 900 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0