the manner in witch you do it is pretty much the same no matter witch mail you use (hotmail, yahoo, ect) i use hotmail so i can tell you how to do it in that.

for hotmail...
in your inbox right click the message that you wanna know where it can from and click "view message source." it should bring you to a new page. the page should have all kinds of crazy stuff on it. the first thing that should be on the page should be "X-Message-Delivery:{then some stuff here...all codey crap...}
you then just look down the page till you see
"X-Originating-IP: []"
and what ever is in the place of is where it came from.

i think that this process is farely similar for other mail providers. you should just have to find the headers, and the origination ip should be in there. if you have troubles finding it then please tell me the mail provider you need help with and i will try my best to find out how it is done for you.