When I reply to a somewhat longer email, I find myself scrolling quite a lot between the original message at the bottom and then back up to where I left off writing my reply message.

While you could open the message in its own window and position and resize it in such a way that it sits nicely next to your compose screen so you can read them both, it is a somewhat cumbersome solution.

Did you know that you could split your message compose window directly from within Outlook?