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?
Migadu doesn’t provide sources for their deployment, as far as I know, but most of the components they use are open and available on the usual places: https://github.com/migadu