_s2.outbound.mailhop.org has no MX records.
Question and response
Question
dig _s2.outbound.mailhop.org. MX
Answer
The response has no answer records.
Authority
mailhop.org. 1800 SOA henry.ns.cloudflare.com. dns.cloudflare.com. 2397539225 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0