mx.terra.com.br has no NS records.
NS records for this zone are defined at terra.com.br.
NS records for this zone are defined at terra.com.br.
Question and response
Question
dig mx.terra.com.br. NS
Answer
The response has no answer records.
Authority
terra.com.br. 720 SOA a.dns.terra.com. sysadm.terra.com.br. 2017046903 300 300 2419200 7200
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0