_netblocks.mimecast.com has no CNAME records.
Question and response
Question
dig _netblocks.mimecast.com. NS
Answer
The response has no answer records.
Authority
mimecast.com. 5479 SOA dns01.mimecast.com. dns-admin.mimecast.com. 1772146955 300 300 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0