mx.online-navi.jp has no CNAME records.
Question and response
Question
dig mx.online-navi.jp. NS
Answer
The response has no answer records.
Authority
online-navi.jp. 3600 SOA asns1.customer.ne.jp. dnshosting.nttpc.co.jp. 1970010166 3600 1800 3600000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0