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.