We pick f.root-servers.net, one of the 13 root servers (1), and ask it about root-servers.cn and learn that cn is delegated to six name servers (2), so we ask b.dns.cn where we learn that root-servers.cn is delegated to three name servers, including ns2.cdns.cn (3), so we ask that one and get the following answer:
Most of the more than hundred domains that use the primary name server ns1.cdns.cn use name server group 1 (example: xy2s.cn, no114.cn and guarder.cn ). Worth noting is that all of the domains that use name server group 1 use the primary name server ns1.cdns.cn.
The mail server mxdomain.qq.com only
There are several thousand of domains that only use the mail server mxdomain.qq.com.