The "email postage" idea isn't a new one (though congrats for thinking of it). The idea has been to charge something fractional like .01 cents/email. People who send a hundred emails a day will only pay 1 penny a day, or less than a dollar a month.
But, folk who send millions will cost more.
The "free to a friend" idea's an interesting one, but wouldn't that require a whole new email system? After all, email is effectively transient: once it's left each email server, it's completely forgotten (gotta love SMTP).
How would you imagine such a system working or being implemented practically?