dns03.mimecast.net has no MX records.
Question and response
Question
dig dns03.mimecast.net. MX
Answer
The response has no answer records.
Authority
mimecast.net. 86400 SOA dns01.mimecast.com. root.mimecast.net. 1770327230 10800 900 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0