grp.grpmail.jp has no CNAME records.
Question and response
Question
dig grp.grpmail.jp. NS
Answer
The response has no answer records.
Authority
grpmail.jp. 2902 SOA ns01.key-p.co.jp. root.ns01.key-p.co.jp. 2026022602 28800 14400 3600000 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0