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“One mistake” would be if he didn’t double-down on it, and if Proton addressed their customers’ concerns in any meaningful way. Instead, they deleted posts and are now withdrawing from the community entirely, and directing users to three of the worst corporate hell-holes on the internet.
Proton does not use end-to-end encryption for email headers. That includes the subject lines, senders/recipients, and other potentially sensitive information.
Tuta uses E2EE for email contents AND headers.
Consider for a moment what someone with access to your contacts and subject lines would know about you. For me personally, they would know which political campaigns and causes I donate to, and when. They would know when I see various doctors, and who they are. They would know my travel dates and destinations. They would know what newsletters I read (many of which are also political). Etc.