This is big news! I’ve held off from moving interi.org over to ProtonMail because I’d need multiple accounts.
I like that they use the name “Susan” in the example account creation, because that would be one of my accounts, too! 
I haven’t fully vetted how ProtonMail works, though I do know it can’t be accessed with standardized clients or protocols, given the nature of its storage. This probably doesn’t matter for most people, but it might matter for me. Can’t wait to check it out!