To understand why this won't work you have to understand how e-mail works. We start from when you hit 'send' in outlook.
Your message first goes to your ISP's or company's outgoing mail server. Let's ignore that for a moment.
That outgoing mail server looks at the recipient- firstname.lastname@example.org. So it uses DNS (the thing that converts a name like www.google.com into an IP like 22.214.171.124) and asks what the MX (mail exchanger) servers are for domain.com. Domain.com has those listed in its DNS.
The outgoing mail server then connects to the domain.com MX server. It says "i have a message from email@example.com for firstname.lastname@example.org". If the MX agrees to take it, your outgoing mail server transmits the message, and the MX sends a confirmation that it is accepted. They then disconnect.
If you're running your own mail server, or are using a company mail server, or a different email system, your ISP has nothing to do with this other than moving your packets around.
The point is that email is not a single system that can be changed like raising the fare on the subway. If you're the city and you want higher subway fares, you just reprogram a few thousand turnstiles (all of which you own) and you're done. Email/SMTP isn't like that, SMTP is an agreement, a protocol which millions of networks and servers have chosen to implement. Email is just another internet protocol, no different than AIM, skype, HTTP/wwww, FTP, etc. It's just one of the most widely used protocols.
There is no central authority to enforce anything like e-stamps. For this to be enforced, the domain.com MX would have to say 'please give me a tenth of a cent before I deliver your mail'. The only useful way to handle that would probably be with a 3rd-party clearinghouse for exchanging the 'stamps', so your mail server would say 'i give you stamp ID (long stamp id number)', the destination MX looks that up with the clearinghouse, approves it, then accepts the message for delivery.
For that to happen, both your SMTP server and the recipient's MX would have to be modified to deal with these payments, and optionally require them for mail delivery. There are many different mail server programs out there, this would require all of them to be updated to support payments, and then (heres the hard part) all the people who run them would have to install those updates. Then anybody who runs a mail server would have to do some financial setup to let them accept payments and send payments for email. IE, every random geek and company and IT department and ISP that runs a mail server now has to jump through a financial hoop. If I run my own mail server, does that mean i get 2/3 of the payment (the recipient fee and the ISP fee)? Does my ISP get it even though I'm not using their servers? There will be great resistance to this.
The main issue is, it would *NOT* be transparent, not to anybody. This would be a large, time-consuming and very expensive implementation.
Now let's say best case scenario, lets say you get all the major isps and webmail providers on board (msn, aol, yahoo, google, comcast, timewarner, verizon, cablevision/optimum, charter, adelphia, etc).
Let's say they immediately set up their system to start dealing with these micropayments.
What happens to the (literally) millions of companies in the US and abroad who run thier own mail servers, but whos systems are NOT updated? Can they no longer send mail to all of the above networks, or is there a break in period? If the payments are optional, what incentive does anybody have to adopt them?
Also you say approved senders can send for free. Who is an approved sender? What is the qualification? If it's difficult and expensive, some of the large bulk-mailing companies will try it anyway, and the smaller legit companies are shut out. If it's easy to get one even for a small biz, then the spammers will get them too. If extensive investigation is performed on the applicants, that money has to come from somewhere, so it'll be expensive. And whoever you put in charge of this all will undoubtedly want to make money off it so the price may well go up over time. No matter what you do, a lot of people are going to be very unhappy about this.
Say you have a 'free' list for each subscriber. Is this maintained on the server? Great, now the MX server which used to just relay mail has to maintain allow lists for thousands of subscribers. Expensive.
You say that the people with infected computers would patch if it cost them money-- no they wouldnt. Many of those machines are in asia where nobody cares and everybody runs pirated software. And for the US side ones, you're talking about having ISPs start packet inspecting their traffic to look for email to bill for. This is a BAD idea.
I agree with you that tiny e-stamps might help the spam problem. But I just don't see it happening anytime this century. One of the biggest principles of the Internet is that anybody can send any kind of data across it at any time for any reason. SMTP (email) is just another type of data, just another protocol. You can't just change it, even if you have a good reason for doing so it's just not physically possible.