mx1.opxtest.com has no NS records.
NS records for this zone are defined at opxtest.com.
NS records for this zone are defined at opxtest.com.
Question and response
Question
dig mx1.opxtest.com. NS
Answer
The response has no answer records.
Authority
opxtest.com. 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