sysmail2.linecorp.com has no NS records.
NS records for this zone are defined at linecorp.com.
NS records for this zone are defined at linecorp.com.
Question and response
Question
dig sysmail2.linecorp.com. NS
Answer
The response has no answer records.
Authority
linecorp.com. 600 SOA ns2.naver.jp. domain.linecorp.com. 1772077958 3538 600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0