Open source:
- Mailpile (https://www.mailpile.is/)
- PEPS (https://github.com/MLstate/PEPS)
Proprietary:
- Protonmail (https://protonmail.ch/)
and https://lavaboom.com/ (also open source I think)
Tutanota, Whiteout and Lavaboom are also all from Germany, which I find pretty interesting.
You know the joke about how to apply for a job at the NSA? Most of the parents at my childrens' school would get that.
My temporary solution is to combine endpoint encryption (eg GPG), MyKolab for address/storage, air gaps, and a guard. The MyKolab account gets me Swiss storage with associated legal protection & lack of clever Google-style snooping. I assume the servers are compromised along with messages. To deal with those threats, people send me either GPG messages or otherwise encrypted files. For protection, I can download them to a disposable, hardened PC; send them through a guard or data diode for reading; use a separate computer for writing and signing with a data diode. This is Markus Ottela's architecture for Tinfoil Chat. His diodes with separate PC's are simpler than my guards with KVM-connected PC's. So I recommend it his way these days.
You can swap out MyKolab for any other service for delivery or storage. You just have to make sure they're totally untrusted, incoming messages can't compromise your keys, and keys/secrets can't leak out. Tricky stuff for any of these developers. TFC already does this. I suggest these people modify its latest incarnation to do email (maybe apply GPG), find any other flaws it has, and improve on docs/distribution. Will get more mileage.
Do you mean a PC booted with a non-persistent OS like Tails or a Linux Live CD?
How would you rate a Qubes VM for this purpose?
You just want these systems hardened from attack as much as possible along with ease of detection and easy recovery. The disposable part means exactly what it says: the Internet-connected computer is the target and filter of the most risky functionality. It will be toast at some point, maybe often. So, use a throwaway device for it.
Mailpile does not host mail, it is just the front end - you still need an email account elsewhere.
I use runbox.com as my main email provider - not free (pricewise), but is doesn't cost much.