> Mail is stored e2ee on server

Exclusively, or do they keep caches around? I am asking since everything is clear text in the webmail. I wonder if they handle the rare case of proton to proton (encrypted) mail differently from regular unencrypted mail. I assume they have to decrypt a master key stored on the server with your password, and then decrypt every encrypted email on the fly on the server, or they have to send the master key to the client side.

Now think that through when you have thousands of searchable e-mails, sorted arbitrarily. I won't say it is impossible, but I think that maintaining plain text indexes rather than encrypted ones are really tempting.