Apex domains cannot have CNAME records.
Question and response
Question
dig email.microsoftonline.com. NS
Answer
email.microsoftonline.com. 5 NS ns2.exacttarget.com.
email.microsoftonline.com. 5 NS ns1.exacttarget.com.
email.microsoftonline.com. 5 NS ns3.exacttarget.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0