I've also been a happy customer for about a year now.
edit: maybe they are? http://oxpedia.org/wiki/index.php?title=SourceCodeAccess Why is there no link on main website?
Also, the document editing is commercial license with 30-day trial license only.
Their commercial license start at 5000€
I'm genuinely curious if ProtonMail has similar functionality on offer, especially since it appears to be free.
[1] https://support-en.mailbox.org/knowledge-base/article/the-en...
[2] https://mailbox.org/en/ensuring-emails-are-sent-securely/
edit: links
TL;DR: It encrypts just about everything in storage but "Subject lines and recipient/sender email addresses are encrypted, but not end-to-end encrypted.", which tells me they might have access to these things.