ns15.digicertdns.net has no MX records.
Question and response
Question
dig ns15.digicertdns.net. MX
Answer
The response has no answer records.
Authority
digicertdns.net. 180 SOA ns10.dnsmadeeasy.com. dns.dnsmadeeasy.com. 2009010122 43200 3600 1209600 180
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0