Apex domains cannot have CNAME records.
Question and response
Question
dig imismailcenter.com. NS
Answer
imismailcenter.com. 5 NS pdns2.ultradns.net.
imismailcenter.com. 5 NS pdns6.ultradns.co.uk.
imismailcenter.com. 5 NS pdns4.ultradns.org.
imismailcenter.com. 5 NS pdns5.ultradns.info.
imismailcenter.com. 5 NS pdns1.ultradns.net.
imismailcenter.com. 5 NS pdns3.ultradns.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0