Tell HN: Proton is reading and filtering incoming mail

This seems relevant to privacy-conscious folks here:

I just tried to use my existing ProtonMail account as a backup for my Gmail account. Instead of receiving the verification code, I got an email in my Proton inbox that says:

"Your Proton account is currently restricted from registering on third-party services"

Meaning that proton is actively reading (and filtering) mail from certain flagged accounts. Otherwise, how do they know my account just received a matching email from Google with a verification code?

This is not particularly surprising for most providers, but I was not expecting it from Proton. What's the point of all the encryption claims?