v50.v5756819d.use4.send.mailgun.net has no NS records.
NS records for this zone are defined at v5756819d.use4.send.mailgun.net.
NS records for this zone are defined at v5756819d.use4.send.mailgun.net.
Question and response
Question
dig v50.v5756819d.use4.send.mailgun.net. NS
Answer
The response has no answer records.
Authority
v5756819d.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