Apex domains cannot have CNAME records.
Question and response
Question
dig v576a36ee.use4.send.mailgun.net. NS
Answer
v576a36ee.use4.send.mailgun.net. 5 NS ns-cloud-e4.googledomains.com.
v576a36ee.use4.send.mailgun.net. 5 NS ns-cloud-e3.googledomains.com.
v576a36ee.use4.send.mailgun.net. 5 NS ns-cloud-e1.googledomains.com.
v576a36ee.use4.send.mailgun.net. 5 NS ns-cloud-e2.googledomains.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0