Apex domains cannot have CNAME records.
Question and response
Question
dig mail.mil. NS
Answer
mail.mil. 4 NS utindo02.csd.disa.mil.
mail.mil. 4 NS sdns3.csd.disa.mil.
mail.mil. 4 NS guv10m01.csd.disa.mil.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0