Every email would be digitally signed and encrypted. Certificate with keys would connected to email address (and bitcoin wallet).
Spam would die.
Go build it please.
I keep thinking instead about something that works more like a refundable bond:
To send a message to you, I have to include some amount. (I don't know how much. Not a penny, though. Maybe $10 or $100.) If you want to keep receiving messages from me afterward, you return the bond. If not, you keep the money.
And after a couple of minutes of Googling, I just found a name for schemes like this: Attention Bonds.
Maybe it would just become more targeted?
In any case, I would love to only send/receive encrypted/signed email. I don't think the problems are technical.
http://craphound.com/spamsolutions.txt
and check all boxes that apply.
The bitcoin transaction can just go to /dev/null with a hash of the message as data, so don't have to pay somebody ( ;) ) and you cannot track who sent data to whom via tracking the bitcoin transactions.
Other than that, I agree paying to send will reduce spam.
But also look at your postal mail box. There is arguably more spam there than in your digital inbox, and that one costs (stamps).
Are folks afraid of bitcoin? What can be worse than just having a 16 digit number printed on a card -- that is a 1000x more vulnerable.
You are part of the tiny percentage of the tiny percentage of the population that understands bitcoin.