sysmail1.linecorp.com has no CNAME records.
Question and response
Question
dig sysmail1.linecorp.com. NS
Answer
The response has no answer records.
Authority
linecorp.com. 236 SOA ns2.naver.jp. domain.linecorp.com. 1772071939 3538 600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0