mailgate.so-net.ne.jp has no MX records.
Question and response
Question
dig mailgate.so-net.ne.jp. MX
Answer
The response has no answer records.
Authority
so-net.ne.jp. 3600 SOA zns.so-net.ne.jp. hostmaster.so-net.ne.jp. 2026021802 3600 300 3600000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0