mx.mitene.jp has no CNAME records.
Question and response
Question
dig mx.mitene.jp. NS
Answer
The response has no answer records.
Authority
mitene.jp. 158 SOA nsx1.mitene.ad.jp. postmaster.po.mitene.or.jp. 101125 7200 1800 1209600 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0