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?
Not really much point. Email as a protocol isn’t secure, so they can always use some software to read the messages in transit even if they secure them at rest. Many systems use Dovecot, Postfix, and other systems that have been open for ages. Only the UI is likely proprietary and that’s mostly just for viewing and billing. And anything they’re doing that’s nefarious is likely a separate code base entirely since it doesn’t need integration with the data in motion not being secure.
What are you looking to gain by understanding their code base?