mx.core.locaweb.com.br has no CNAME records.
Question and response
Question
dig mx.core.locaweb.com.br. NS
Answer
The response has no answer records.
Authority
core.locaweb.com.br. 3600 SOA ns1.locaweb.com.br. hostmaster.locaweb.com.br. 2022260901 3600 600 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0