Thunderbird hashcash pluginMozilla mail and thunderbird do not yet support hashcash directly but there is a thunderbird plugin (click thunderbird at left) that supports hashcash.
hashcash support in mozilla mail / thunderbird
what features would make sense? receive support - Mozilla mail and thunderbird include a bayesian learning anti-spam feature. In my personal experience it makes a fair number of false positives. It would help if mozilla supported hashcash on received mail.
send support - It would help if mozilla supported sending hashcash so that mozilla mail users could benefit from the spamassassin support, TMDA support and other anti-spam systems which are starting to support hashcash as a way to bypass their filters to make sure your mail gets delivered and does not get falsely classified.
stamp dialog - Hashcash can be a somewhat lengthy process on slow hardware (1 20-bit stamp per second on a 600Mhz CPU; 3 20-bit stamps per second on a 3.06Ghz P4 (see here for more benchmarks)). It would be useful if some progress indicator were given.
background stamp creation - in offline mode mozilla could compute stamps queued ready for sending.
precomputed stamp pool - mozilla could precompute in the background stamps for recipients you frequently communicate with.
early start to stamp creation - mozilla could start to compute stamps as soon as you click on reply, or enter recipients into To, Cc, Bcc.
minimal supportActually receive support alone to avoid false positive from mozilla's own spam filter would I think be rather easy to implement as verification is very cheap and simple.
However it would seem a bit one sided to try to benefit from receive support without participating in send support. Minimal could be a progress dialog on send. Or maybe not even. How slow a machine can you comfortably run an app the size of mozilla on? Note even rather slow machines by current standards such as 300Mhz P2 can create a stamp in 2 seconds.