This might make it easier to understand:
With HKDNR you register one or more name servers. These can be with a hosting company, like yahoo.com or one of your own machines sitting in a data center somewhere.
That machine then points to a name where mail should be delivered. Than name can be one of your own, e.g. mail.foo.hk, or it can be with a email hosting company, e.g. mail.yahoo.com.