mail.fxo.com.br has no NS records.
NS records for this zone are defined at fxo.com.br.
NS records for this zone are defined at fxo.com.br.
Question and response
Question
dig mail.fxo.com.br. NS
Answer
The response has no answer records.
Authority
fxo.com.br. 900 SOA ns-1680.awsdns-18.co.uk. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0