v52.v51c4a094.usw1.send.mailgun.net has no NS records.
NS records for this zone are defined at v51c4a094.usw1.send.mailgun.net.
NS records for this zone are defined at v51c4a094.usw1.send.mailgun.net.
Question and response
Question
dig v52.v51c4a094.usw1.send.mailgun.net. NS
Answer
The response has no answer records.
Authority
v51c4a094.usw1.send.mailgun.net. 300 SOA ns-cloud-a1.googledomains.com. cloud-dns-hostmaster.google.com. 1 21600 3600 259200 300
Additional
. 0 OPT ; payload 512, xrcode 0, version 0, flags 0