This is an instance moderation problem. If you’re letting spammers in, you need to use a better application process or something similar to that. A big problem with email spam is that most email services allow anyone to sign up for free without any checks.

Which is one reason, this author is arguing, that e-mail has become so centralized. Doing that kind of manual moderation and curation is expensive, the bigger instances out-compete the smaller ones who don't have as much resources to dedicate to it. As more and more instances get "de-federated" for not having as good of anti-spam measures as the bigger instances, more users will sign up at big instances to avoid defederation risk. Just like how many people use gmail simply because their email delivery rate is so good. If I send from g-mail, there's very few servers which will reject my message or throw it in the spam folder. I'd love to run my own mail server, but even as a dedicated sysadmin it's impossible to get decent delivery rates.

The more anti-spam checks we have, yes we weed out spam, but we also make it accessible to less users as well.

AP has been blessed so far with not having to fight too much spam. Look at very popular, very centralized, very resourced platforms like Facebook, spam is still a problem on their platform despite massive resources put towards fighting it.