using hashcash in procmail
First you will need to install the hashcash tool if it is not included with your distribution.
This came from Kyle Hasselbacher's example:
which should deliver to DEFAULT anything with a valid X-Hashcash: header addressed to email@example.com.
More advanced stuff (multiple addresses):
which should deliver to DEFAULT anything *@*.hasselbacher.org or *@*.toehold.com. The $MATCH is the stuff after the / so you can use procmail regexps to specify what email addresses are valid for you.