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?
The few I’ve seen were so small and new that I forgot about them. I remember one detail though, he was developing a jmap based email client and it’s code was on GitHub. On the server side he was using Stalwart.