Penny Post is a system that helps combat spam emails. Using Penny Post, the sender "pays for email" by spending a few seconds of computing time. Outgoing mail is automatically marked with postage stamps that convince the receiver that the sender is genuine.
The Thunderbird extension makes it a breeze to stamp email and verify stamps, plus it looks cool. Try it!
The latest release for Thunderbird is a bug fixing release. Existing users are requested to upgrade asap.