Apex domains cannot have CNAME records.
Question and response
Question
dig emailsrvr.com. NS
Answer
emailsrvr.com. 5 NS ns.rackspace.com.
emailsrvr.com. 5 NS ns2.rackspace.com.
emailsrvr.com. 5 NS pdns219.ultradns.biz.
emailsrvr.com. 5 NS pdns219.ultradns.com.
emailsrvr.com. 5 NS pdns219.ultradns.net.
emailsrvr.com. 5 NS pdns219.ultradns.org.
Authority
The response has no authority records.
Additional
ns.rackspace.com. 5 A 69.20.95.4
ns2.rackspace.com. 5 A 65.61.188.4
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0