Good idea; I wish there were a way to implement it.
As pointed out by others, you can't intercept an email on its way to tax it, but you can always catch it at its final destination. Here is an idea that doesn't involve a government tax.
As you rightfully point out, I bear some cost for each message I receive, so I could pass on this cost to the sender by demanding a fee for reading the message. Essentially this would need a credit system where my incoming mail server can silently drop any message that doesn't come with a small electronic payment of some form.
For any person with whom I have a normal, symmetric communication, this just cancels out, but those who want to spam me would have to pay for that priviledge.
Obviously, implementing such a credit system to be both reliable and inexpensive is a major technical challenge, but certainly something that I'd like to see explored.