mx.a.cloudfilter.net has no MX records.
Question and response
Question
dig mx.a.cloudfilter.net. MX
Answer
The response has no answer records.
Authority
mx.a.cloudfilter.net. 900 SOA ns-1804.awsdns-33.co.uk. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0