mail.coreserver.jp has no NS records.
NS records for this zone are defined at coreserver.jp.
NS records for this zone are defined at coreserver.jp.
Question and response
Question
dig mail.coreserver.jp. NS
Answer
The response has no answer records.
Authority
coreserver.jp. 1800 SOA gabriel.ns.cloudflare.com. dns.cloudflare.com. 2396973762 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0