Outlook's "Message Options..." menu displays a message's header. Unfortunately, there doesn't seem to be a keyboard shortcut to access this menu.
1. Double-Click on the Quick Access Toolbar's "Customize" icon. (It's almost invisible.)
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2. Select the "More Commands..." menu item
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3a. Open the "Choose Commands From..." drop-down menu
3b Select the item "All Commands"
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4a. Scroll way down in the "Customize" menu
4b. Select the item "Message Options..."
5. Select the "Add >>>" button to add the item to the right-hand "Customize Quick Access Toolbar" pane
6. Select OK
1. Select (single-click highlight) the desired message. Do
not open (double click) the message.
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2. In the "Quick Access Toolbar", select the "Message Options..." icon, which opens a (mis-titled) "Properties" popup window.
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3. Scroll through the headers in the lower pane of the "Properties" popup window.