If you still have the whole message, including the header, you might ask your ISP to investigate. Some states have laws on the books about hate mail in general and some have or are considering specific extensions to email. The ISP may be able to determine how your connection was traced and (possibly) where the email came from. In any case, if you feel threatened or compromised you should report the incident to authorities.
One more thing, was there anything in the email that you could say uniquely identified (targetted) you individually? Or was it more general and could have applied to anyone who had voiced your kinds of opinions. That would help an investigation.