alpha-sendmail.cos.navercorp.com has no MX records.
Question and response
Question
dig alpha-sendmail.cos.navercorp.com. MX
Answer
The response has no answer records.
Authority
navercorp.com. 180 SOA ns1.naver.com. webmaster.navercorp.com. 2026022501 21600 1800 1209600 180
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0