v538.v594dfbfc.use4.send.mailgun.net has no NS records.
NS records for this zone are defined at v594dfbfc.use4.send.mailgun.net.
NS records for this zone are defined at v594dfbfc.use4.send.mailgun.net.
Question and response
Question
dig v538.v594dfbfc.use4.send.mailgun.net. NS
Answer
The response has no answer records.
Authority
v594dfbfc.use4.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