I get these all the time... what has ha;ppened is that you have your email online in a message, on a chat room, in a website... the choices are endless, and a "namebot" harvested it... now it's being used to "spoof" messages.
Yes, you CAN track this down, but only if you've the resources of the NSA or sonething equally large. You'd need programmers, a VERY fast and large computer and- lets' face it- that ain't you.
Report this to Yahoo an change your email. Let your friends know your new email (do NOT use an automatic mail-change notification program) and then be very careful about where you leave traces of yourself. My actual email is shown only as a .GIF image on webpages and the 'bots can't pick those up (yet)!
Pretty sad when theese dorks screw up what should be a great experience, but if there's a way to make money illegally, you know someone's gonna be there in a flash.
Change the email and just be careful in future... and good luck.