It's easy to stop spam. Just buy everything they try to sell you. Eventually, the people selling amazingly cheap mortgages and viagra and spy-on-your-spouse services and all the other other fine products and services will be so rich they will retire and stop sending you spam! 🙂
But seriously, this is not a get-richer-quick scheme by Microsoft. It is a legitimate suggestion for a way to combat spam, and there aren't many other plans with a chance of success.
I don't like the idea of having solve-a-math-puzzle be the way to pay postage, since it favors big-bucks businesses too much over regular folks. So hard cash is the way to buy e-mail postage. Note that there has to be a foolproof way to prove that a message has postage on it, which I assume would be with digital signatures from a postal authority you choose to trust.
Nobody would be forced to participate in such a program. If you want to accept e-mail with no postage, you can continue to do so. You determine your tolerance level. Many people would choose to allow incoming messages only if they had postage or were on a personal whitelist (a junk mail rule that says "accept mail from this particular source").
I suggest something else that would help too: Right now, people can put a "bookmark this page" button or a "make this your home page" button on a web page. With the same ease of use, I'd like to see web browsers and e-mail clients develop a feature where you could click "whitelist this e-mail address" or "whitelist this domain" to add a rule to the junk mail filter with a simple click and a confirmation dialog box. That way, when you sign up for yet another e-mail service or online forum, you would do something simple to exempt that site from being filtered by your junk mail filter and its "postage" setting.