SirDice. Look up Eigth Wonder they have/had a spoofer using Hotmail. They used to have a server (mail.hotmail.com I think) you could telnet into and send mail from. Maybe it is still up but I have not tried to use it in a while. Yahoo! probably has or had the same type of thing. In the end headers can tell you most everything, although if they sent it from Yahoo! or what not you are going to have to have them check the logs which they probably won't do.

-Cheers-