Could you implement this whitelist on your side by simply having two inboxes? E-mails from addresses in your whitelist go to "Messages from known people", other e-mails go to "Messages from unknown people".

This wouldn't require any special action from other people. You would automatically be a bit more suspicious when looking into the "unknown people" inbox, and if you verify the author, you add their name into whitelist and automatically move all their e-mailt to the "known people" inbox.

You could also set it up this way for your less tech-savvy relatives, and ask them to call you immediately if they find something in the "unknown people" inbox.