I think you confused E2EE with Star Wars robot R2D2 :P

Ultimately, even when E2EE isn't used, the mailbox access is still fully encrypted. ProtonMail has to accept mail in whatever form on the ingress side, but once it's received, it's encrypted with your credentials. Something GMail or Outlook doesn't do at all as your mails are essentially stored there in plain text, only access to that plain text format is granted through correct password which is just for user side access, not for the operator aka Google. Only point they are encrypted is during transit from their mail server to another mail server. It's why GMail can do all sorts of clever mail sorting, combing and data extraction because they have all the access to your mails and ProtonMail cannot because they don't have such access. Only access they have is some metadata, mail addresses and subject lines for basic spam filtering and sorting. They have no access to mail body.

One could argue one could spy on users on the ingress side, but when you're that paranoid, you really need to ensure all participants in communication use E2EE using same clients, be it voice, mail or IM. Ultimately, even if ProtonMail wasn't encrypted, I'd use it because my primary goal was just stop using GMail. There are paid for services that claim to respect user privacy as you pay for the service where Google's "all this free shit" has to cost something else if it's free as service.