Version:            (using KDE 4.3.1)
OS:                Linux
Installed from:    SuSE RPMs

An explanation can be read in the kdepim archives: http://lists.kde.org/?l=kde-pim&m=124195221523397&w=2

Basically this is about kmail doing the wrong thing when the spam filter tool is not sure whether a message is spam or ham.

There are two things that mus tnot happen:
1. If unsure, do not mark a message as spam or ham.
2. If unsure do not do nothing.

2. is very important to get spam tools going, i.e. to train them, especially the light-weight ones that do not slow down kmail. If kmail does not do anything but keep them in the inbox, the user won't act on ham, because there is no need to mark anything as ham that is already in the correct folder, i.e. the inbox.

Since the user does not mark any emails as ham, the spam filtering tool cannot learn what is regarded as ham and thus not decide what should be marked as spam.

If kmail would stick to the spam filtering tool's definition, i.e. move emails that it is not sure about to an unsure folder, the user would act upn those emails and mark them as either spam or ham thus training the spam filtering tool.