Apex domains cannot have CNAME records.
Question and response
Question
dig bbmail.com.hk. NS
Answer
bbmail.com.hk. 5 NS ns1.hkbn.net.
bbmail.com.hk. 5 NS ns2.hkbn.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0