s21.coreserver.jp(202.172.28.22) が Barracuda のブラックリスト入りしており、
メール送信(相手先受信)に不都合がありました。
一先ず、個人的に解除を申請しており回答を得ています。

回答抜粋ここから
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Thank you for contacting Barracuda Networks regarding your issue. Your issue is important to us. We have assigned a confirmation number: BBR21400560726-80713-10304 to this case.

We apologize for any inconvenience that this may have caused you. We have removed 202.172.28.22 from our blocklist for 30 days, at which time it will be re-evaluated.

There are a number of reasons your IP address may have been listed as "poor", including:

1. The email server at this IP address contains a virus and has been sending out spam
2. The email server at this IP address may be configured incorrectly
3. The PC at this IP address may be infected with a virus or botnet software program
4. An individual in the organization at this IP address may have a PC infected with a virus or botnet program
5. This IP address may be a dynamic IP address which was previously utilized by a known spammer
6. The marketing department of a company at this IP address may be sending out bulk emails that do not comply with the CAN-SPAM Act
7. This IP address may have a insecure wireless network attached to it which could allow unknown users to use it's network connection to send out bulk email
8. In some rare cases, your recipients' Barracuda Spam Firewall may be misconfigured

If you do not think any of the above apply, please also contact the person who manages this IP address, as they may be better able to investigate this issue.

Thank you for your time and understanding.

Barracuda Central by Barracuda Networks
http://www.barracudacentral.com/
[email protected]

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回答抜粋ここまで

しかしながら、度々こういった事が起こると面倒ですので、
同サーバに、スパムや要因があるようでしたら排除をお願いしたいと思います。
お忙しいところ済みませんが宜しくお願い致します。

slake様

BRBLへの解除申請をいただき有難うございます。

s21.coreserver.jp(202.172.28.22)から3月14日にSPAMの配信がございましたが、既に対応しており、それ以降、現在までSPAMは確認しておりません。
このため、登録要因につきましては、私どもでは判りかねます。

先方様の送信ガイドラインに従って配送いただくこととホワイトリストに追加いただくことをお願いしております。