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.
Okay, so how do you bootstrap a new server in that system?
What do you do when you just created a server and can't get new users because you aren't whitelisted yet?
But what if you do handful of users to start out, or just yourself? How do become 'active' without being able to federate with any other servers? Talk with yourself?