mail.coreserver.jp has no CNAME records.
Question and response
Question
dig mail.coreserver.jp. NS
Answer
The response has no answer records.
Authority
coreserver.jp. 1736 SOA gabriel.ns.cloudflare.com. dns.cloudflare.com. 2396973762 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0