Example from Yahoo!'s Header information:
This would be the ip of the person sending the mail. If your address is a yahoo address, this is what it would look like. It might help you figure out which number you are looking for.
X-Originating-IP: [xxx.xxx.xx.xx]
Edit
In the case of Outlook (I just did this test to myself), the number you would be looking for should be the second
Recieved From in the header.