Email is an open system. Anyone can send an email to someone else if they know their address.
An email system following these rules:
1. Only messages from an email address on your whitelist get to you.
2. Any other email is bounced back automatically.
3. You can put email addresses of others on your white list by hand.
4. You can also do so by sending them an email from your address.
5. You can give the other person a one-time token to send you (in the subject for instance) that would automatically put them on your white list permanently, or only allow this one email to pass through to you, and then you can decide to give them permanent or temporary (up to X number of emails, X amount of time, combination of these, etc.)