dolphin.kddnet.ad.jp has no MX records.
Question and response
Question
dig dolphin.kddnet.ad.jp. MX
Answer
The response has no answer records.
Authority
kddnet.ad.jp. 300 SOA penguin.kddnet.ad.jp. sysadmin.noc.kddnet.ad.jp. 2017061901 600 180 3600000 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0