Apex domains cannot have CNAME records.
Question and response
Question
dig mktgc.salesforce.com. NS
Answer
mktgc.salesforce.com. 4 NS dns1.p09.nsone.net.
mktgc.salesforce.com. 4 NS dns2.p09.nsone.net.
mktgc.salesforce.com. 4 NS dns3.p09.nsone.net.
mktgc.salesforce.com. 4 NS dns4.p09.nsone.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0