Protonmail = privacy at all costs. Thundermail = freedom at all costs (open standards, use w/ whatever client you want, upload your own encryption key and "go dark")

I assume if I upload my own key, the receiver of the emails needs to also have my key? I'm not necessarily trying to send encrypted emails to people. I would want zero-access encryption which means not even Thundermail knows what my emails are.

No, we rely on PGP encrypted email. What I'm talking about is the latter. Emails are encrypted with your public key before being stored in the database. (Our stack is based on Stalwart, you can read about how it works here: https://stalw.art/docs/encryption/overview )

Thanks for explaining the broad strokes. I'm a bit more interested now and signed up on the waitlist.