jmap-server VS ProtonMail Web Client

That would be amazing. You even started with the JMAP server?

It is used by Fastmail (who was also developers of the standard)

I think it is not a great idea to say that it is not a good idea to not implement things that no one uses, because that is a certain way to not make more people use it.

There are some servers that support it such as Stalwart [0], though I don't know about any older servers and/or clients that support it well.

[0]: https://github.com/stalwartlabs/jmap-server

Email Server: Stalwart JMAP Server, Stalwart JMAP Client, Stalwart MailSend, and pretty much anything else from Stalwart

Today Stalwart JMAP v0.2 was released including support for the for JMAP for Sieve Scripts draft. Additionally, ManageSieve support was added to Stalwart IMAP v0.2.

The source code is here https://github.com/ProtonMail/WebClients

> Finally, in keeping with our long track record of transparency, Proton Pass is open source so anyone can review and verify our security architecture

They sure do enjoy writing that sentence without including any hyperlinks. This (https://github.com/ProtonMail/WebClients/tree/main/applicati...) appears to be the browser extension and https://github.com/ProtonMail/WebClients/tree/main/packages/... appears to look like the backend referenced in the extension's readme, but that directory's readme is zero bytes so (shrug)

Fork the frontend and make your own lightweight option

PS: I hope that we selfhosters will have a modern, efficient, easy to use mail suite one day with modern features like JMAP, good self-learning spam integration, automated checks and validations for SPF/DMARC/DKIM or whether the IP/host suddenly appears in a blocklist and integrated encryption at rest for emails. Something that isn't 30 services in a container image, with 30 different configuration styles. Maybe even with an API integrated that's compatible to the ProtonMail frontend (like the neutron server once intended to be). Anyway, I'm sorry for dreaming. ;)

And if you want to customize it further you can use Stylus to add custom CSS, Tampermonkey to add JS, or even modify the whole thing yourself from source (if you run it locally it syncs with your actual account).

There are lots of useful End to end encrypted webapps such as Protonmail, cryptpad, cryptee and many others. And part of why we trust them is that the client side code is open source, so we can see that end to end encryption is really going on and is really secure. Its impossible to verify what code they are running in their server, but if the data we send them is already encrypted we can rest easy.