ns4.dns-server.jp has no MX records.
Question and response
Question
dig ns4.dns-server.jp. MX
Answer
The response has no answer records.
Authority
dns-server.jp. 600 SOA ns1.dns-server.jp. domain.seeds.ne.jp. 2025032601 3600 900 3600000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0