sysmail2.linecorp.com has no CNAME records.
Question and response
Question
dig sysmail2.linecorp.com. NS
Answer
The response has no answer records.
Authority
linecorp.com. 50 SOA ns2.naver.jp. domain.linecorp.com. 1772087559 3538 600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0