To add hashcash to outgoing mail with many unix mailers go read about Kyle Hasselbacher's hashcash-sendmail package. Another copy on archive org (www.toehold.com seemed down mar 2010).

You will also need to install the hashcash tool if it's not already included with your unix distribution

what does it do?

hashcash-sendmail actually starts a background process to work on generating your stamps and then delivers your message when this has finished. So it will return instantly but your message will not actually get sent until the necessary stamps are generated.

integrate with your mail system

Then make your mailer invoke hashcash-sendmail instead of system sendmail, however you do that on your system. Below is a list of mailers with instructions for how to invoke hashcash-sendmail for specific unix MUAs:

mutt
see also Kyle's hashcash-sendmail page as he also has a list of systems with configuration instructions
email me adam@cypherspace.org or the hashcash-list if you have questions about or suggestions on how to do this with other unix mailers.

another mutt hashcash script

There is also Tim Ruddick's foreground hashcash filter for mutt. This creates hashcash in the foreground rather than backgrounding a process, and then sends using sendmail.