I use mailbox.org. Mailbox.org provides an "encrypted mailbox" feature, which PGP encrypts incoming unencrypted emails. The server can of course intercept incoming messages, but it can't look at the entire backlog unless it was compromised the entire time.
Alternatively, using POP3 instead of IMAP (at least with the default settings) deletes emails from the server after downloading, whenever my laptop is connected. Thus, the server can intercept incoming messages, but not the entire backlog.
Of course, both of these have downsides. The encrypted mailbox is PGP, so it misses important details like the subject lines and source addresses. Meanwhile, POP3 can leave my mail entirely unprotected for as long as I'm offline, and it also means that I can't access it from anything other than my laptop, and means that I have to do manual backups.
Which is more important in terms of security, or should I use both? I'm looking for the legal perspective of law enforcement (In Canada and Germany, home to myself and my email provider respectively), but also that of some hacker who's trying to get into my (and everyone else's) accounts.
Would there be a server software that I could use to download emails from mailbox.org over POP3 and then provide them to all my own devices over IMAP? That might, in some sense be the best of both worlds. Right now, I am using both POP3 and the encrypted mailbox, but convenience is definitely not optimal, so I'd like to change if it can be done safely.
If that's your concern, I would move to a zero-knowledge provider.
Edit: although you should remember that the email in transit is not encrypted, so am attacker could sit in front of your provider and read every message in plain text.
Do you know of any zero-knowledge providers that are both (a) trustworthy for my own purposes, and (b) unlikely to go to spam?
Like you said, the incoming messages aren't encrypted, so "zero-knowledge" is always sort of false advertising. Also, if I have to use some weird client, that isn't good. I do value convenience, especially for email; chasing diminishing returns just isn't worthwhile, and if possible I'd like to not use both, as I am now.
You can use your own pgp keys w mailbox. And 3rd party email clients. IMO that's the best of both worlds for the available providers that support pgp.