How can I trace the anonymous sender of an email using a yahoo.com
address. I have a copy of the email which was actually sent to third
parties and have a good idea who the person is but need to prove it in
order to initiate action.

I've found this info pretty handy..
http://www.usus.org/elements/tracing.htm

I work for an email provider. gotta talk to your email provider so
that they can grab the ip address that hit there server when they sent
you that email. thats the only way since he couldve forged his email
address.. so im pretty sure its not from yahoo.