mailserver.sinet.ad.jp has no CNAME records.
Question and response
Question
dig mailserver.sinet.ad.jp. NS
Answer
The response has no answer records.
Authority
sinet.ad.jp. 180 SOA dns01.sinet.ad.jp. postmaster.sinet.ad.jp. 2026022001 60 900 259200 180
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0