Apex domains cannot have CNAME records.
Question and response
Question
dig mimecast.com. NS
Answer
mimecast.com. 5 NS dns02.mimecast.net.
mimecast.com. 5 NS dns03.mimecast.net.
mimecast.com. 5 NS dns04.mimecast.net.
mimecast.com. 5 NS dns01.mimecast.net.
Authority
The response has no authority records.
Additional
dns01.mimecast.net. 1 A 185.58.87.12
dns02.mimecast.net. 1 A 205.139.110.125
dns03.mimecast.net. 1 A 195.130.217.24
dns04.mimecast.net. 1 A 170.10.128.103
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0