KDE's Project Silk

Even if you only store three emails in it, it still makes sense to make
them accessible for your applications other than the primary email client.
You also don't want to rewrite all the email protocols over and over, for
every new email application. That's what Akonadi takes out of your hand.

As I said, an sqlite backend is under way. That would be the one catering
to those very simple use cases you're talking about. It has only become an
option very recently, due to technical issues with it (see other comment).