Proton retains your data definitively, and although it is encrypted, they’re the ones who generate your encryption key as well. If they want to act maliciously, they are totally able to. Also can’t register using a VPN or Tor without giving personable identifiable info. Tutanota has the second problem as well.

Skiff collects the pages you visited before using their webmail - creepy:

Automatic Data Collection. In order to protect you and our platform from malicious activity and to prevent fraud, we may collect certain information automatically when you use our Services, such as your Internet protocol (IP) address (temporarily), user settings, and Skiff-provided authentication cookies. We may also temporarily collect information regarding your use of our Services, such as pages that you visit before, during and after using our Services, the frequency and duration of your activities, and other information about how you use our Services.