I did a test this morning from my Yahoo account sent via the web to my Gmail account. When viewing the message header, about 3/4 of the way down, I see a line that says:
Received: from [xx.xx.xx.xx] by web55704.mail.re3.yahoo.com via HTTP;
The IP in brackets is my home IP address.
Looks like the originating IP can be traced in Yahoo mail.