It's become used for person-to-person communications, as well as computer-to-person communications, and is probably one of the few standard, interoperable, near-ubiquitous means of reaching people, short of postal mail and carrier-provided services like SMS and voice calling.

While requiring people to know who they want to correspond with before the fact could be helpful, it also breaks a lot of popular use-cases for email - email is often written into contracts as a valid means of serving notice etc pursuant to the contract. Locking email down to only receive mail from approved senders would break a lot of things, but certainly help to prevent this kind of spam.

I wonder to what extent this could be reduced however by using unique per-use aliases - in a sense you can get a softer version of this whitelist by giving out per-sender inbox aliases, and therefore seeing which alias was used to send a message. Not quite sender authentication, but perhaps better than nothing. Doesn't prevent compromise of the underlying (human-to-human) email address, but would certainly help with a lot of phishing-type scenarios using breached customer records.

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.