Apex domains cannot have CNAME records.
Question and response
Question
dig mailcloud.com.cn. NS
Answer
mailcloud.com.cn. 5 NS ns3.dnsv2.com.
mailcloud.com.cn. 5 NS ns4.dnsv2.com.
Authority
The response has no authority records.
Additional
ns3.dnsv2.com. 5 A 1.12.0.29
ns3.dnsv2.com. 5 A 1.13.76.55
ns3.dnsv2.com. 5 A 111.13.203.55
ns3.dnsv2.com. 5 A 117.135.128.155
ns3.dnsv2.com. 5 A 125.94.59.155
ns3.dnsv2.com. 5 A 163.177.5.55
ns3.dnsv2.com. 5 A 220.196.136.55
ns3.dnsv2.com. 5 AAAA 2402:4e00:1470:2:0:0:0:12
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0