c0.c74c62ca.usw1.send.mailgun.net has no NS records.
NS records for this zone are defined at c74c62ca.usw1.send.mailgun.net.
NS records for this zone are defined at c74c62ca.usw1.send.mailgun.net.
Question and response
Question
dig c0.c74c62ca.usw1.send.mailgun.net. NS
Answer
The response has no answer records.
Authority
c74c62ca.usw1.send.mailgun.net. 300 SOA ns-cloud-b1.googledomains.com. cloud-dns-hostmaster.google.com. 1 21600 3600 259200 300
Additional
. 0 OPT ; payload 512, xrcode 0, version 0, flags 0