Apex domains cannot have CNAME records.
Question and response
Question
dig mim.ec. NS
Answer
mim.ec. 1 NS dns02.mimecast.com.
mim.ec. 1 NS dns03.mimecast.com.
mim.ec. 1 NS dns04.mimecast.com.
mim.ec. 1 NS dns01.mimecast.com.
Authority
The response has no authority records.
Additional
dns01.mimecast.com. 4 A 185.58.87.12
dns02.mimecast.com. 4 A 205.139.110.125
dns03.mimecast.com. 4 A 195.130.217.24
dns04.mimecast.com. 4 A 170.10.128.103
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0