ns01.usen.ad.jp has no MX records.
Question and response
Question
dig ns01.usen.ad.jp. MX
Answer
The response has no answer records.
Authority
usen.ad.jp. 3600 SOA ns03.usen.ad.jp. root.ns03.usen.ad.jp. 2017011000 10800 3600 3600000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0