mx.dooray.com has no NS records.
NS records for this zone are defined at dooray.com.
NS records for this zone are defined at dooray.com.
Question and response
Question
dig mx.dooray.com. NS
Answer
The response has no answer records.
Authority
dooray.com. 180 SOA a1-57.akam.net. webmaster.toastoven.net. 2026020901 21600 1800 1209600 180
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0