The host at this IP address is being (ab)used to «listbomb» email addresses:
From: シックス・アパート株式会社 <no-reply@sixapart.com>
Subject: MT 7 r.5003 / 6.8.3 / Premium 1.47 の提供を開始（セキュリティアップデート）【Movable Type News Letter 2021年10月号】
Problem description
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Spammers signed up for the bulk email service using the victim’s email address. As a result, the victim is being «listbombed» with transactional messages and bulk email campaigns.
Problem resolution
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In order to resolve this spam problem (and have this SBL listing removed), the affected sender must take the folloing actions:
a) Implementing CAPTCHA to prevent automated subscriptions
b) Implementing Confirmed Opt In (COI) if not already done so
c) Clean up their email address list (e.g. sending out a permission pass / COI)
Further reading
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Further information can be found on the referenced links below.
Subscription Bombing: COI, CAPTCHA, and the Next Generation of Mail Bombs:
https://www.spamhaus.org/news/article/734/
Mailing Lists -vs- Spam Lists:
https://www.spamhaus.org/whitepapers/mailinglists/
Confirmed Opt In — A Rose by Any Name:
https://www.spamhaus.org/news/article/635
Spamhaus Marketing FAQ:
https://www.spamhaus.org/faq/section/Marketing%20FAQs