Tuta and Proton, which are both commonly recommended, have not released their back-end code (read about it here and here). Proton doesn’t seem interested in open sourcing their code, and Tuta has claimed they’re aiming to do it in the future (the claim was made years ago and nothing has happened yet). They also both claim to be open source in their marketing.
Is there any alternative that is fully FOSS?
Something like Disroot? Just about all the services they host are done with FOSS software. For email I think (?) they’re using Nextcloud and RoundCube. In theory you could look for other providers that are using that software stack, there are paid service/hosting providers that provide Nextcloud hosting for example.