Every message, contact, meeting and appointment you open in Outlook has its own window. As you work away in Outlook during the day you might find your Taskbar cluttered with lots of Outlook windows.
Want a quick way to declutter and close them all? Use the Close All Items button. It’s the last button on the View tab at the extreme right of the Ribbon. If you have unsaved work such as draft emails, Outlook prompts you to save them.
I use this frequently enough that I’ve added it to my Quick Access Toolbar.