Index pictures of text
I often see people posting screenshots of text (code, console output, etc) instead of the text itself. Later on, when wanting to search in email for details about this email, the text in the picture is completely unsearchable. It should be relatively straightforward to run OCR on an image, to see if contains text, and if so, index it like normal text elsewhere in the email.
If that's how reliable client-side OCR is these days, I hope all those Recaptchas helped.