Others have already said similar, but it depends on what you mean by "private" and "secure". Yes, proton is e2e but that only is true if you're emailing another proton user. And yeah, Proton can't read your emails, but as soon as you email someone else outside the ecosystem, it's as good as public.

I'm not saying thou shalt not use proton! But I had a subscription and cancelled. One part because the CEO vocally supported Trump and the doubled down when called out. Another part because I got tired of the proton ecosystem being inaccessible outside its own walled garden because of the e2e.

So I switched to Fastmail and couldn't be happier. Is it perfect, no, but what is? It works well, it's reasonably priced, they've been around forever, and I can use it with apps/programs outside of what they provide.

When it comes down to it, your email will never be truly private unless you only communicate with people who are just as concerned about privacy.