server.validsender.com has no MX records.
Question and response
Question
dig server.validsender.com. MX
Answer
The response has no answer records.
Authority
validsender.com. 1800 SOA kim.ns.cloudflare.com. dns.cloudflare.com. 2397109283 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0