Mx.castam.com.br has no CNAME records.
Question and response
Question
dig Mx.castam.com.br. NS
Answer
The response has no answer records.
Authority
castam.com.br. 6 SOA ns978.hostgator.com.br. root.br978.hostgator.com.br. 2026021501 86400 7200 3600000 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0