Outlook 98, Outlook 2000, Outlook 2003

1. Open the message in a separate window (double click)
2. Under the View menu select Options
3. Copy the text in the Internet Headers window (unfortunately it doesn't include the message itself).
4. Paste
5. Close the options window

Outlook 2007

1. In main list, click right on the email, where you want to view the header
2. In the list select Message Options
3. Copy the text in the Internet Headers window (unfortunately it doesn't include the message itself).
4. Paste
5. Close the options window

Outlook 2010, Outlook 2012

1. Open the message for which you want to view the headers
2. In the message window, select the File tab
3. Click the Properties button
4. Copy the text in the Internet Headers window (unfortunately it doesn't include the message itself).
5. Paste
6. Close the Properties window