Version: 1.7.2 (using KDE 3.3.2, compiled sources)
Compiler: gcc version 3.3.3 (SuSE Linux)
OS: Linux (i686) release 2.6.5-7.111.19-default
When applying filters to the mailbox, the application is completely locked, no busy-mousecursor is displayed (clock), and no cancel option is provided.
This would be important since filter operations can take a very long time (just ran a filter over a mailbox with several thousand messages). For such operations, either a cancel option/progress dialog should be provided, and/or they should run in the background. At least, the mouse cursor should indicate that the application is busy.
You missed one of the highest rated KDE bug reports when looking for existing duplicates. Consider voting for http://bugs.kde.org/show_bug.cgi?id=41514
*** This bug has been marked as a duplicate of 41514 ***