herokudns.com has no MX records.
Question and response
Question
dig herokudns.com. MX
Answer
The response has no answer records.
Authority
herokudns.com. 10 SOA dns1.p05.nsone.net. hostmaster.nsone.net. 1661188672 600 900 1209600 10
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0