[programminggeek]

So, if I send an ecrypted protonmail to someone else's yahoo mail, what happens? Is it only encrypted in the protonmail ecosystem?

True end to end encryption would mean everything is transferred as an encypted thing, and only people with a key can open it. If any email you send out ultimately is unencrypted so that the other side can read it, we aren't much closer than where we started are we?

If an email ends up in an unencrypted IMAP mailbox on a server somewhere, how is that more secure than what happens now?

[x1798DE]

I think it sends them a note that says, "Someone at ProtonMail sent you a message - click this link and enter the password they gave you to open it!"

Presumably they'll have some way to distribute the password in some ephemeral or slightly out-of-band way. It's probably less secure than messages within their environment, but it shouldn't ever hit another mailserver in plaintext (ideally ProtonMail wouldn't even have the plaintext anyway).