1. Open the email in a new window by double-clicking it.
2. Click the expansion button in the lower right corner of the Options toolbar button.
3. The box by default holds the Categorize & Mark as Unread buttons.
4. Find the headers under "Internet Headers" at the bottom of the dialog box
To View All Message Headers in Outlook 2000, 2002 & 2003:
1. Open email in Outlook
2. Click View > Options
3. All header lines appear under Internet Headers at the bottom of the dialog that comes up that displays something like DKIM-Authentication-Results
To View All Message Headers in Outlook 2010:
1. Open email in Outook
2. Click File tab, beside the Message tab
3. Click Info
4. Click Properties
5. The Properties pop-up would show the Internet Headers at the bottom of the window.