v51a3e4cb.usw1.send.mailgun.net has no MX records.
Question and response
Question
dig v51a3e4cb.usw1.send.mailgun.net. MX
Answer
The response has no answer records.
Authority
v51a3e4cb.usw1.send.mailgun.net. 300 SOA ns-cloud-e1.googledomains.com. cloud-dns-hostmaster.google.com. 1 21600 3600 259200 300
Additional
. 0 OPT ; payload 512, xrcode 0, version 0, flags 0