Please. Captcha by default. Email domain filters. Auto-block federation from servers that don't respect. By default. Urgent.
And yes, to refute some comments, this publication is being upvoted by bots. A single computer was needed, not "thousands of dollars" spent.
What does this mean?
In other words, for every server that does not adhere to the other rules mentioned (captchas, email requirements, etc), automatically de-federate from all of them.
If you're not familiar with what "de-federate" means, check out this article on federation concepts