Comment by palata
> The last two providers offered true end-to-end encryption
This is not quite right. The only offer e2ee if you send an email to someone on the same provider (e.g. ProtonMail to ProtonMail). If you write to someone using Gmail, it's not e2ee.
IMHO this kind of e2ee is interesting for companies (because every employee is on the same provider, and it's better to have the internal communications on ProtonMail than shared with Google on Gmail), but for a personal email it doesn't matter so much.
What's really important is to have a custom domain so that you are not stuck with one provider.
That's correct, I meant that you have to always use their app, whether to use e2ee or not. There is no IMAP.
Mailbox also offers e2ee via browser among the same mailbox users, but it also has IMAP and PGP.