> If both sides use ProtonMail, communication is end to end secure.
If both sides use [the same provider], it's not mail anymore, it's something internal. That is the fundamental issue of ProtonMail/Tutanota/any service provider that pretends to solve end-to-end encryption in email: without standards, it's a proprietary system. Today the only viable path towards easy E2E encryption in email is Autocrypt. AFAIK only Posteo is working towards including it.
If both sides use [the same provider], it's not mail anymore, it's something internal. That is the fundamental issue of ProtonMail/Tutanota/any service provider that pretends to solve end-to-end encryption in email: without standards, it's a proprietary system. Today the only viable path towards easy E2E encryption in email is Autocrypt. AFAIK only Posteo is working towards including it.