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 <email@example.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】
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.
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 information can be found on the referenced links below.
Subscription Bombing: COI, CAPTCHA, and the Next Generation of Mail Bombs:
Mailing Lists -vs- Spam Lists:
Confirmed Opt In — A Rose by Any Name:
Spamhaus Marketing FAQ: