If you want to have a quick look at your diary or tasks list whilst in the middle of an email, you can do this using Sneak a Peak. Just hover your mouse over the Calendar, People or Tasks shortcuts on the Navigation Bar at the bottom of the Outlook window.
Hovering over the Calendar will show you a preview of your appointments for the day. Hovering over Tasks will show you tasks that are scheduled. Hovering over People will show you people that you have added to your Favorites.
If you like this features, you can also dock a sneak peek of your calendar and tasks to the Outlook window so that they are always visible – just click the Dock button in the upper right corner of the peek. This will dock the peek to right side of the current view.