Problem, Emails can only be searched by the subject or the fixed header info neither of which can be altered by the user on incoming mail. We all know this isnt sufficient for many searches, but also understand the end to end encryption challenges.

Simple Solution, Id propose a single button or link on each email received or created that pops up a field where the user can type or paste in text meant to be searchable. That would be a quick fix for being able to find important emails, and of course the user would have to understand that data would not be stored encrypted any more than the subject line is now, at least not under their private key, but maybe under a proton known key so it can be searched but still reasonably secure from compromise.

Maybe not for the uber paranoid user, but for the average person and business user not fearing a subpoena release of this content it would be a very useful feature.
Just an easy way to quickly cut and paste a paragraph, add an account number etc. to be more quickly found later. I would use this on many more important emails that don't have reasonably descriptive and searchable subject lines.