Apex domains cannot have CNAME records.
Question and response
Question
dig mx1.emailsrvr.com. NS
Answer
mx1.emailsrvr.com. 5 NS ns2.rackspace.com.
mx1.emailsrvr.com. 5 NS pdns219.ultradns.com.
mx1.emailsrvr.com. 5 NS pdns219.ultradns.org.
mx1.emailsrvr.com. 5 NS pdns219.ultradns.net.
mx1.emailsrvr.com. 5 NS pdns219.ultradns.biz.
mx1.emailsrvr.com. 5 NS ns.rackspace.com.
Authority
The response has no authority records.
Additional
ns.rackspace.com. 4 A 69.20.95.4
ns2.rackspace.com. 4 A 65.61.188.4
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0