[inigyou]

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.

[JimDabell]

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.