Full indexing: <https://lesbonscomptes.com/recoll>, <https://userbase.kde.org/Akonadi>, <https://addons.mozilla.org/firefox/addon/falcon_extension> (If you're not content with a piece, then research substitutes on <https://alternativeto.net>.)
Recent: `.local/share/recently-used.xbel`
This does not help with the email part because email programs do not register opened messages in recently used. Work-around: install a DBus or AT-SPI hook and write your own database of recently opened messages.
Happy hacking!