I recently left ProtonMail and went back to Fastmail. My reason was that they will never be able to fully support IMAP and now CalDAV because of the encryption they use. I grew to accept that email is not for secure messaging and my paranoia of "I'm being watched" just went away. If you need secure messaging, use something other than email.
I came to a similar conclusion. You should write every email as if it were public, because it's entirely likely that it will be. They can be forwarded, made public through legal discovery, or exposed in a data breach (eg. Sony/North Korea). Forget security for a second, imagining every email as public record will make you more considerate and less biased writer. And from a business perspective, email should be viewed…
> Just because I consider every email I write to be public
Cool. Can I have the creds to your Fastmail account then? I'm curious what you're up to these days. If you're not comfortable doing that (I promise I won't send any emails - not that it matters, I could still spoof your address anyway), I could settle with a dump of all the emails you've sent.
I get the whole 'write every email with the mindset that it could one day end up on the front page of NY Times' but that doesn't dismiss security requirements for locking down your email/calendar account.