@Gargron Still the problem is Mastodon. See github.com/mastodon/mastodon/d.

Please see these issues (two of them are created by me and are related) as well:

*Require blocking of disposable email providers and/or require a captcha provider when registrations are open*

github.com/mastodon/mastodon/i

*Set new registrations on new servers to manual approval by default*

github.com/mastodon/mastodon/i

*Ability to greylist new servers*

github.com/mastodon/mastodon/i

*Ability to use heuristic spam filtering tools*

github.com/mastodon/mastodon/i

*Instance-wide filtering*

github.com/mastodon/mastodon/i

cc @renchap