Apex domains cannot have CNAME records.
Question and response
Question
dig global.exacttarget.com. NS
Answer
global.exacttarget.com. 5 NS gtm02.exacttarget.com.
global.exacttarget.com. 5 NS gtm01.exacttarget.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0