Not so fast with the young horses

[ghost]

Monitor and review registrations from IP addresses with bad reputation (open proxy servers, Tor exit nodes), OR enforce additional checks on those users, for example by requesting a CAPTCHA or verifying the user's phone number.

Anyone who has ever run an email server knows that spam is a tricky thing. Spam is an unsolicited commercial offer. And the decision to receive spam or not is solely up to the recipient.
There are court rulings that have forbidden email server operators to filter out spam for the end customer.
The only thing we can do is to reduce the number of messages for financial reasons. Slow down, deliver later or never.
For this we need to get together and write a protocol. How many messages per second a user is allowed to send or receive, that can be determined by the server. You have to collect all possible approaches and compare them against each other, and then create a strategy.
That's what it has to come down to in the end.