ns1.dnsimple.com has no MX records.
Question and response
Question
dig ns1.dnsimple.com. MX
Answer
The response has no answer records.
Authority
dnsimple.com. 3600 SOA ns1.dnsimple.com. admin.dnsimple.com. 1438186502 86400 7200 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0