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?
Someone has to pay the bills, so they need to earn some money. If you ain’t paying you are the product is always a good metric, but in the email case the problem is also that a lot of the effort is manly IT admin rather than just the backend software.
You can roll your own dovecot and run your own emails at home in theory, in practice you need to be trusted by the major email providers to be able to send emails not tagged as malicious by anyone.
To protect their trusted status most providers collaborate on a lot of elements to be mutually sure they are not flooding each other with worthless spam. This is a lot of SPF, and other technical bullshit to be able to run a mail provider someone has to pay for
In case it is not clear: most use foss backend (dovecot or other), but this will not cover the whole software stack needed for an email provider
To clarify, this isn’t about selfhosting or wanting a free service. I already pay for an email provider and I have no intention of hosting one myself. This is only about principles.