Apex domains cannot have CNAME records.
Question and response
Question
dig smtp2go.com. NS
Answer
smtp2go.com. 5 NS ns4.smtpcorp.com.
smtp2go.com. 5 NS sns4.treshna.co.nz.
smtp2go.com. 5 NS ns2.smtpcorp.com.
smtp2go.com. 5 NS sns3.treshna.co.nz.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0