Proton Mail

Details

The service provided is not open source, or source available. The email clients are provided under a GPLv3 license, not the service itself.

The project advertises its service as a whole as open source, as reflected on its website:

Open source and independently audited
All Proton services are open source and independently audited for security.

There are several threads on GitHub querying the server-side sources (1, 2, 3). The Proton CTO responded in one thread, whose comments included the following:

I don’t think we’ll be doing this any time soon, for a couple reasons. […]

[…] there’s no trust advantage here because the what is running on the servers is fundamentally unverifiable by the client.

[…] Enabling self-hosting or giving a leg up to would-be competitors by open-sourcing our entire stack doesn’t seem like the right move to protect our own viability as a company.

Proton services are operated by Proton AG, a Swiss corporation whose primary shareholder is the non-profit Proton Foundation.

The project appears to funded from donations, grants, merchandise and from its paid hosted services.