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.
There are literally globally maintained blacklists of spam email sources. When people lease a static IP address the first thing to do is to check it against the major email blacklists.