dns4.istweb.com.br has no MX records.
Question and response
Question
dig dns4.istweb.com.br. MX
Answer
The response has no answer records.
Authority
istweb.com.br. 86400 SOA ns280.hostgator.com.br. root.br280.hostgator.com.br. 2026013101 86400 7200 3600000 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0