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.