We’ve now limited 40 domains due to recent spam coming from them. All these instances have open registrations and automatic approvals.
Having automatic approval makes your instance vulnerable to scripted spam attacks. If you haven’t realized that by now, you willingly put other instances and users at risk.
Unfortunately Mastodon only provides very limited moderation tools to counter these kind of attacks. Until then, please take on some responsibility and switch to manual approvals for your instance.
Edit: 62 domains
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