mailgw.oneoffice.jp has no MX records.
Question and response
Question
dig mailgw.oneoffice.jp. MX
Answer
The response has no answer records.
Authority
oneoffice.jp. 900 SOA dns1.broadline.ne.jp. dns-info.broadline.ne.jp. 668 10800 900 604800 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0