File README.SUSE of Package hashcash

hashcash-sendmail can be invoked in place of /usr/bin/sendmail when used for
enqueueing email. It will compute a hashcash checksum and insert an X-Hashcash
header line before submitting it using the real /usr/bin/sendmail.

To configure e. g. mutt for using hashcash-sendmail, use the following line in your
$HOME/.muttrc:
set sendmail='/usr/bin/hashcash-sendmail'