Version: 1.12.2 (using KDE 4.3.2) OS: Linux Installed from: Mandriva RPMs For folder lists, the vertical scroll bar handle size is inverse proportional to the number of messages in the list, and so becomes invisible for message lists with a large number of messages. There should be a minimum size set (e.g. 10px or a percentage of the the full height) for the scroll bar handle.
This is probably a style problem. Please try e.g. with Plastique (which I use and don't see this problem, using a mailbox with > 10000 mails).
Martin, I can confirm that the problem is caused by the IaOra-KDE widget style; it is not present with any of the other styles. Thanks for the reply, best wishes, John