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For starters, open up the entire email in its raw format, and look at *all* the headers. See if anything looks familiar. look up any IPs or domains you find in there (besides Yahoo, I know you already tried that), check the provenance of addresses, etc.
If the harrassor has done due diligence anonymizing themselves, you won't be able to identify them... they're hiding behind Yahoo, and sadly those anonymous email hosts can be very effective shelters.
Examine the tone, grammar, and content of the email, see if anything raises any identifying flags. you say they obviously know you... can you guess how they know you? What do they say that suggests familiarity?
(I'm suggesting you consider these questions, but please do not post excepts from the email here)
I'm hesitant to suggest this, but... reply to the email with an obscure hotlinked image in the HTML body. There is a slight chance that the sender may open the email, may display the image, and then there'll be a record of that request in your server logs, possibly with a real IP. Which could give you a stupidly vague guess at their location. It's a real long shot.
Harassment is illegal. People do go to jail for doing that. If the emails are threatening, call the police and ask them for advice.
disclaimer: IANAL - I am Not a Lawyer