dns01.mimecast.com has no NS records.
NS records for this zone are defined at mimecast.com.
NS records for this zone are defined at mimecast.com.
Question and response
Question
dig dns01.mimecast.com. NS
Answer
The response has no answer records.
Authority
mimecast.com. 86400 SOA dns01.mimecast.com. dns-admin.mimecast.com. 1773291139 300 300 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0