ns12.customer.ne.jp has no MX records.
Question and response
Question
dig ns12.customer.ne.jp. MX
Answer
The response has no answer records.
Authority
customer.ne.jp. 86400 SOA ns1.sphere.ad.jp. dns.sphere.ad.jp. 2026022601 10800 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0