ns02.herokudns.net has no MX records.
Question and response
Question
dig ns02.herokudns.net. MX
Answer
The response has no answer records.
Authority
herokudns.net. 3600 SOA dns1.p01.nsone.net. hostmaster.nsone.net. 1664479343 43200 7200 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0