Apex domains cannot have CNAME records.
Question and response
Question
dig mimecast.Co.Za. NS
Answer
mimecast.Co.Za. 1 NS dns02.mimecast.com.
mimecast.Co.Za. 1 NS dns03.mimecast.com.
mimecast.Co.Za. 1 NS dns04.mimecast.com.
mimecast.Co.Za. 1 NS dns01.mimecast.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0