Apex domains cannot have CNAME records.
Question and response
Question
dig a.sharedmail.jp. NS
Answer
a.sharedmail.jp. 5 NS ns3.dns-server.jp.
a.sharedmail.jp. 5 NS ns4.dns-server.jp.
a.sharedmail.jp. 5 NS ns1.dns-server.jp.
a.sharedmail.jp. 5 NS ns2.dns-server.jp.
Authority
The response has no authority records.
Additional
ns1.dns-server.jp. 5 A 210.239.65.38
ns2.dns-server.jp. 5 A 18.177.45.193
ns3.dns-server.jp. 5 A 210.152.8.223
ns4.dns-server.jp. 5 A 15.168.90.255
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0