Look at the Received headers on the message to you. These should be in reverse chronological order (most recent at the top). You can generally trust the topmost headers (your own ISP, etc.). At some point there is a header you don't trust. That header and everything below it may be a forgery.
If someone forges an email to fake their identity, and tricks you into violating a court order by responding to what appears to be legitimate mail, they, not you are the responsible party. Include the entire email with headers in your reply to the suspected email, and there should be no problem in establishing what happened.
I've heard of phishing for profit, phishing to scam credit card numbers. This is the first time I've heard of "phishing for revenge".
