mx.dooray.com has no CNAME records.
Question and response
Question
dig mx.dooray.com. NS
Answer
The response has no answer records.
Authority
dooray.com. 103 SOA a1-57.akam.net. webmaster.toastoven.net. 2026020901 21600 1800 1209600 180
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0