alt Hacker NewsHey.com works that way. You have to approve new senders before they can reach your inbox. And you can always revoke their permission to message you.
I'd like to see that concept replicated to other email services. I don't particularly like all the other opinionated choices of Hey.com (especially the fact that you can't use IMAP).
This sounds to me like a normal black/white list, but everything is on the blacklist by default.
I imagine this can be achieved with most mailboxes with a simple deny all rule and then cherry picking email addresses to whitelist.