b.dns.usp.br has no MX records.
Question and response
Question
dig b.dns.usp.br. MX
Answer
The response has no answer records.
Authority
dns.usp.br. 300 SOA a.dns.usp.br. hostmaster.usp.br. 2018053341 10800 3600 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0