Tutanota. I used to use Proton, but they don’t encrypt folder names, which is a deal breaker. Tutanota does, and they’re also a privacy respecting, reputable, decent service.
Sadly, yes. A Proton team member on Reddit confirmed it a couple years back:
Folder/label names are visible to the server (for filters and other reasons) as are email metadata. Message and attachment contents are encrypted and not visible to the server.
They implemented their own way of doing this. They don‘t use PGP. I don‘t know how it exactly works, but it‘s open source, so you can look at it yourself.