What does hashcash do?
Hashcash comes in the form of plugin software for mailers which adds hashcash stamps to sent email.
The hashcash plugin software inserts a X-Hashcash: header into the email headers section of the email the user sends. The following is an example of an email addressed to me with a hashcash stamp in the email headers:
this is stupid. posting stamps doesn't work for everything you know HAHAA
From: Someone <email@example.com>
To: Adam Back <firstname.lastname@example.org>
Subject: test hashcash
Date: Thu, 26 Jun 2003 11:59:59 +0000
What stops a spammer using hashcash?
Spammers can use hashcash too, however hashcash is bad news for spammers because the hashcash stamp takes your CPU some work to compute. To you as a normal user, with an entry level desktop or laptop class machine the CPU overhead per mail is negligible because you don't send that many mails; at worst your mail is delayed a few seconds before being sent on slow old hardware. However to spammers this is a show-stopper: they want to send 10,000+ emails per minute down a DSL line bought with a stolen credit card quick before the account gets cancelled.
But won't spammers steal CPU time?
...etc etc etc