Apex domains cannot have CNAME records.
Question and response
Question
dig vtigermail.com. NS
Answer
vtigermail.com. 2 NS pdns02.domaincontrol.com.
vtigermail.com. 2 NS pdns01.domaincontrol.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0