I like the idea.
Sell it to “famous” people as a way to kill their inbox pain while simultaneously giving to charity and building good PR. Win-win scenario.
As a “regular guy” the first thing that came to my head was “man everyone (meaning everyone that doesn’t have a business mindset) will think I’m such a d*ck if I ask them for $5 just to send me an email.”
Seems like your ideal customer would be someone that has HUGE demands on their time, i.e. the person is famous enough that the majority of emailers know that this person would be doing them a huge favor to respond and who see the value in paying a few dollars for that time. Coincidentally, those “famous” people are also likely to be the same folks that would feel the pain of email-time-suck the most acutely.
Now, that could actually be a big win, because if you tailor your marketing and product to “famous” people, if you get one or two to sign up, you’d immediately have a really large network to refer from. Since folks with large audiences are likely not in need of extra cash, it may mean that the selling proposition is that it’s a way to donate to charity, boost your PR, and filter your email in a guilt-free way.
And if you wanted to go the complete opposite direction…i.e. more toward regular consumers, I think it was Fred Wilson in his post about the possibilities of BTC that mentioned a similar concept based more around anti-spam than making email worth your time (though maybe same problem for different values of $MY_HOURLY_RATE). In any case, the idea was to build an email client that would only accept emails if a certain amount of BTC were attached to it. It would use a trivial amount of BTC, i.e. fractions of a penny that would be negligible to those that aren’t bulk-emailing, but would be enough to threaten the economics of SPAM.