Spam is one of the internet problems that I see on a daily basis in my inbox and if you’re using mail, you probably know the problem. I think one project that is getting far too little attention is Hashcash. The idea is very simple. Email senders put a stamp on their email (which takes a lot of cpu time to compute). Email receivers can validate that stamp (which is cheap), and if there is a stamp, the email is unlikely to be spam. When enough people are using hashcash, mails without stamps can simply be filtered more aggressively. Of course this depends on people using hashcash at all – I took some time to install it recently, which was relatively easy with my Debian/mutt setup. However, things could be made more simple if mailers and distributions came with builtin support.