Nothing at all; it's a broken model. The server can at any time start serving malicious payloads [0]. The server hosts your mail but they also serve the webapp. The clientside decrypts the mail, but the server hosts the client code...

It's a fundamentally flawed idea, trying to retrofit encryption into email in this way, when the server essentially has to hold all of your mail. In this case, the only thing that would make me feel secure in using it, is a third-party OSS client that downloads the mail without using the webapp, using only client-side code. And even then, of course, the mail can simply just be not encrypted when being ingested by Proton. So even then I wouldn't really trust it without external encryption like PGP. In which case, why bother?

To be clear I do use private email services (protonmail, tutanota) but I am simply not going to fall for the illusion of guaranteed privacy; I just trust that they are what they say they are. They are still a better option IMO than something like Gmail, but I don't think they're a silver bullet.

[0]: If you think this is unlikely, see this: https://news.ycombinator.com/item?id=25337507