mx.uol.com.br has no NS records.
NS records for this zone are defined at uol.com.br.
NS records for this zone are defined at uol.com.br.
Question and response
Question
dig mx.uol.com.br. NS
Answer
The response has no answer records.
Authority
uol.com.br. 60 SOA a10-dns-uolcsfe1.host.intranet. root.uol.com.br. 2016057279 7200 3600 432000 60
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0