When the message pane is enabled and one opens a folder, the first message in the list (or the last message that was selected before leaving that folder) is automatically selected and opened. This is rather unwise, since this way messages might get automatically marked as read (=> one then needs to mark them back as unread if necessary).
Moreover, unrecognised spam messages might be opened and external content might be loaded should the sender be faked as someone from the user’s address book. Such a behaviour of eM Client might then be viewed as a security problem.
Considering the fact that this was sort of a requested feature (see https://discourse.emclient.com/emclient/to…), I would suggest an option be added e.g. to Settings > Mail > Read > Read so that users can choose whether to automatically select and open the first/previously selected message upon opening a folder or whether no message should be selected/opened.