hashcash support in kmail
Kmail does not yet support hashcash.However there is a feature request and you can vote to show your interest (you need to create a bugzilla account before you can vote, easy just give email, password).
what features would make sense? send support - It would help if kmail supported sending hashcash so that kmail 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 - kmail could compute stamps queued ready for sending in the background.
precomputed stamp pool - kmail could precompute in the background stamps for recipients you frequently communicate with.
early start to stamp creation - kmail could start to compute stamps as soon as you click on reply, or enter recipients into To, Cc, Bcc.