Apex domains cannot have CNAME records.
Question and response
Question
dig mandrillapp.com. NS
Answer
mandrillapp.com. 5 NS ns-289.awsdns-36.com.
mandrillapp.com. 5 NS ns-720.awsdns-26.net.
mandrillapp.com. 5 NS ns-1127.awsdns-12.org.
mandrillapp.com. 5 NS ns-1934.awsdns-49.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0