Apex domains cannot have CNAME records.
Question and response
Question
dig grpmail.jp. NS
Answer
grpmail.jp. 5 NS ns02.key-p.co.jp.
grpmail.jp. 5 NS ns01.key-p.co.jp.
Authority
The response has no authority records.
Additional
ns01.key-p.co.jp. 5 A 121.83.217.174
ns02.key-p.co.jp. 5 A 122.218.100.83
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0