mgk.cybermail.jp has no MX records.
Question and response
Question
dig mgk.cybermail.jp. MX
Answer
The response has no answer records.
Authority
cybermail.jp. 300 SOA ns1.p202.dns.oraclecloud.net. hostmaster.cybermail.jp. 336 3600 600 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0