I like the idea but wouldn't that also make a lot of legitimate use cases of email too costly? Think of a service like MailChimp.com: They allow you to easily manage and send out newsletters. Computing the hashes for 5000 newsletter receivers would take approximately 5 minutes of raw CPU power.
Also: The CPU power needed has to increase every year because CPUs get better and better (and thus cheaper).
Then again: Let's assume Hashcash would eliminate spam or reduce it: This would probably mean a production boost (no humans have to detect spam anymore) and spam filters cost CPU as well...
In the end it may be a 0-sum game but reverse the burden. The more I think about it the smaller the potential problems seem to be...
Hashcash is actually used by some spamfilters, but not as a sole determining factor. For legit services like MaichChimp, they could be whitelisted (so always clear spam filters) or they can get a bonus to their rating (negative? whichever way would make it less likely to get marked as spam) and then determined to be spam on its contents rather than the absence of a Hashcash value.
Also: The CPU power needed has to increase every year because CPUs get better and better (and thus cheaper).
Then again: Let's assume Hashcash would eliminate spam or reduce it: This would probably mean a production boost (no humans have to detect spam anymore) and spam filters cost CPU as well...
In the end it may be a 0-sum game but reverse the burden. The more I think about it the smaller the potential problems seem to be...