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I remember there was a recent paper that dealt with this, too, but I can't find it right now. But it basically demolished this argument that it's impossible to stop spam if the email is encrypted.

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That blog post doesn't actually say anything about how they do spam filtering on encrypted emails. Everything they talk about is bog-standard spam filtering, including comparing against known spam and Bayesian filtering, both of which are impossible to do on encrypted emails. It's not clear if they're doing reputation analysis, which is the other big anti-spam measure impossible on encrypted email (I suspect it's the main component of their "we're not telling you what we're doing"). The SPF/DKIM/DMARC and blacklist checks are the only things you can reliably do on encrypted email.

They do say "In future blog posts, we will cover the challenges of preventing ProtonMail from being used by spammers and the challenges of doing spam filtering with end-to-end encrypted emails which we cannot read", but I can't find any such blog posts.

WhatsApp also had a session at F8 about e2ee spam filtering...

https://developers.facebook.com/videos/f8-2017/how-whatsapp-...

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