mail3.sendershub.com has no MX records.
Question and response
Question
dig mail3.sendershub.com. MX
Answer
The response has no answer records.
Authority
sendershub.com. 1800 SOA clay.ns.cloudflare.com. dns.cloudflare.com. 2397823761 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0