using hashcash sendmail
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:
another mutt hashcash scriptThere 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.