smtp53.shopback.com.br has no MX records.
Question and response
Question
dig smtp53.shopback.com.br. MX
Answer
The response has no answer records.
Authority
shopback.com.br. 1800 SOA harleigh.ns.cloudflare.com. dns.cloudflare.com. 2395416027 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0