In Microsoft Outlook (I'm using 2013) I often have to click the Check Names button on the ribbon. The Check Names button looks in the address book and validates the name you have entered on the To, CC or BC lines. It's a very useful function but sometimes because of the size of the e-mail window, you can't see the Check Names button easily - it is hidden behind the Names button so it takes two clicks to use it (see the screen shot below).
Because using the button can be cumbersome, I've found the keyboard shortcut very handy.
Click on the To line (or whichever line has the name you want to look up)
More useful Outlook shortcuts can be found here: