Apex domains cannot have CNAME records.
Question and response
Question
dig global-mail.cn. NS
Answer
global-mail.cn. 5 NS ns4.dnsv4.com.
global-mail.cn. 5 NS ns3.dnsv4.com.
Authority
The response has no authority records.
Additional
ns3.dnsv4.com. 5 A 1.12.0.24
ns3.dnsv4.com. 5 A 1.12.0.25
ns3.dnsv4.com. 5 A 1.13.76.53
ns3.dnsv4.com. 5 A 36.155.149.53
ns3.dnsv4.com. 5 A 43.134.249.54
ns3.dnsv4.com. 5 A 124.64.205.153
ns3.dnsv4.com. 5 A 125.94.59.153
ns3.dnsv4.com. 5 AAAA 2402:4e00:1470:2:0:0:0:10
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0