Proton
Empowering you to choose a better internet where privacy is the default. Protect yourself online with Proton Mail, Proton VPN, Proton Calendar, Proton Drive. Proton Pass and SimpleLogin.
Proton Mail is the world's largest secure email provider. Swiss, end-to-end encrypted, private, and free.
Proton VPN is the world’s only open-source, publicly audited, unlimited and free VPN. Swiss-based, no-ads, and no-logs.
Proton Calendar is the world's first end-to-end encrypted calendar that allows you to keep your life private.
Proton Drive is a free end-to-end encrypted cloud storage that allows you to securely backup and share your files. It's open source, publicly audited, and Swiss-based.
Proton Pass Proton Pass is a free and open-source password manager which brings a higher level of security with rigorous end-to-end encryption of all data (including usernames, URLs, notes, and more) and email alias support.
SimpleLogin lets you send and receive emails anonymously via easily-generated unique email aliases.
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2 words I want you to keep in mind:
I purposely do not use the whole Proton suite for those reasons. It would be quite bad if one is entranched by their whole suite if they went on the enshittification route, which large companies eventually tend to do.
Everything in Proton is
Then please do link me the source code of their email protocol.
FairEmail's FAQ is the 1st thing that popped up on Github when searching for "proton", mentioning that it's proprietary:
https://github.com/M66B/FairEmail/blob/master/FAQ.md#faq129
All the Proton software which you can install is open source. It makes sense to have some internal software not revealed.