I wonder if we could get somewhere by asking on thunderbird-dev list. Mike do you know anyone with an interest in anti-spam, hashcash types ofthings from thunderbird side of mozilla?
We want to wrap up this ages old bug mentioned below that has been bouncing around for ages and actually implement something! We can do the hashcash library part we lack thunderbird programming experience. btw something that may help the attractiveness is if hashcash library supports also the microsoft hashcash spec  - its free to use and I thinkopen source compatible even unusually for anything microsoft related. While they basically adaopted hashcash the serialization is incompatible. I'll volunteer to code that spec and add it to hashcash.
hashcash already has both a library (libhashcash.a/hashcash.dll and a command line tool and an example program using the library - example.c) (Note you want to get the pre-release if you want to try the example.c it suffered bit rot I noticed).Adam
 http://download.microsoft.com/download/5/d/d/5dd33fdf-91f5-496d-9884-0a0b0ee698bb/%5BMS-OXPSVAL%5D.pdf On Thu, Mar 31, 2011 at 03:20:43PM +0200, Bas Steendijk wrote:
in the feature request to add hashcash in mozilla thunderbird, https://bugzilla.mozilla.org/show_bug.cgi?id=229686which currently has status "WONTFIX", after i countered the arguments that caused it to get that status, the following happened:i gave argument:"this request is not about adding the 16 MB library, but about adding hashcash, which is simple and can be implemented in little size."Ludovic Hirlimann then posted:"Please provide a patch so we can figure out how much this would add and then decide if we want it in the product or not."i'm asking hereby if someone can produce a patch to add hashcash to thunderbird, and post on there.