mx.gmo.jp has no MX records.
Question and response
Question
dig mx.gmo.jp. MX
Answer
The response has no answer records.
Authority
gmo.jp. 1800 SOA ns1.cf.gmointernet.jp. dns.cloudflare.com. 2396877784 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0