You can do this with email today. Heck, you could do it in 2001, I remember. Hotmail's "exclusive" spam filter policy where anything not from your contacts goes to spam, where you can decide if you want to add them as a contact or not.
That doesn’t work because it relies upon the receiver adding all the possible variations of the sending email address to their address book ahead of time.
That doesn’t work because it relies upon the receiver adding all the possible variations of the sending email address to their address book ahead of time.