mx5.mail.naver.com has no CNAME records.
Question and response
Question
dig mx5.mail.naver.com. NS
Answer
The response has no answer records.
Authority
naver.com. 159 SOA ns1.naver.com. webmaster.naver.com. 2026022501 21600 1800 1209600 180
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0