mx.mattress-direct.com has no CNAME records.
Question and response
Question
dig mx.mattress-direct.com. NS
Answer
The response has no answer records.
Authority
mattress-direct.com. 3600 SOA ns-916.awsdns-50.net. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0