Apex domains cannot have CNAME records.
Question and response
Question
dig secure.emailsrvr.com. NS
Answer
secure.emailsrvr.com. 2 NS pdns219.ultradns.net.
secure.emailsrvr.com. 2 NS ns2.rackspace.com.
secure.emailsrvr.com. 2 NS pdns219.ultradns.biz.
secure.emailsrvr.com. 2 NS ns.rackspace.com.
secure.emailsrvr.com. 2 NS pdns219.ultradns.com.
secure.emailsrvr.com. 2 NS pdns219.ultradns.org.
Authority
The response has no authority records.
Additional
ns.rackspace.com. 2 A 69.20.95.4
ns2.rackspace.com. 2 A 65.61.188.4
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0