Version: (using KDE KDE 3.5.1) Installed from: SuSE RPMs If you double click on a message in a local folder different from the selected one then it is opened in a new window - as the selected one would be. If you do the same in an IMAP folder no new window is opened. The clicked message is loaded and selected only. Thus you have to click once and wait for the message being loaded and have to double click then in order to open it in its own window. I think this can be considered a bug.
Cannot reproduce in KDE 4.3