Hashcash is pure waste, this at least benefits the one whose attention is wasted by the spam.
> You do realise that Bitcoin itself is a complicated protocol built on top of hashcash, right?
Of course, but it already exists.
> If emails require a $1 attachment to get into your inbox, you will receive no emails, since nobody is currently sending any emails with $1 attached.
Yes, but assume there are people sending bulk email legitimately who have delivery issues. All you need is one major provider to accept it and then there is a benefit for senders. DKIM and SPF prove that the deployment problem can be overcome, even for relatively weak attacks on the spam problem.
> You do realise that Bitcoin itself is a complicated protocol built on top of hashcash, right?
Of course, but it already exists.
> If emails require a $1 attachment to get into your inbox, you will receive no emails, since nobody is currently sending any emails with $1 attached.
Yes, but assume there are people sending bulk email legitimately who have delivery issues. All you need is one major provider to accept it and then there is a benefit for senders. DKIM and SPF prove that the deployment problem can be overcome, even for relatively weak attacks on the spam problem.