How can I disable In-Line Reply in the Reading Pane of Outlook 2013

To provide a more streamlined experience without the need of popping up extra message windows and dialogs anymore, Outlook 2013 introduces in-line reply which allows you to press reply and directly start writing your reply in the Reading Pane (which means that it is not really a Reading Pane anymore).

You can press POP OUT to open your message in a regular Message editor window which provides access to additional formatting options and features.

To always use the regular Message editor window instead of in-line reply, enable the option: File -> Options -> Mail -> Replies and Forwards -> Open replies and forward in a new window.