proxy.arena.ne.jp has no MX records.
Question and response
Question
dig proxy.arena.ne.jp. MX
Answer
The response has no answer records.
Authority
arena.ne.jp. 86400 SOA ns3.sphere.ad.jp. dns.arena.ne.jp. 2026012801 10800 3600 604800 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0