_netblocks.mimecast.com has no MX records.
Question and response
Question
dig _netblocks.mimecast.com. MX
Answer
The response has no answer records.
Authority
mimecast.com. 86400 SOA dns01.mimecast.com. dns-admin.mimecast.com. 1771624332 300 300 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0