a0.a0ece303.use4.send.mailgun.net has no CNAME records.
Question and response
Question
dig a0.a0ece303.use4.send.mailgun.net. NS
Answer
The response has no answer records.
Authority
a0ece303.use4.send.mailgun.net. 104 SOA ns-cloud-a1.googledomains.com. cloud-dns-hostmaster.google.com. 1 21600 3600 259200 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0