Short of suing the stalker (whom you do not know I am assuming) there isn't much you can do. If you want to sue, you need to find an attorney who has a background in investigation of Internet cases. He will file a John Doe suit and subpoena the ISP records from the Internet carrier. They should be able to tell him who had the address at the time of the e-mail however if the person is smart he or she acted while in a hot spot and it may not be possible to determine who the exact person was who sent the e-mail. Since the chances of any of this yielding an attorney fee is nil, you will have to pay this person a handsome sum up front. Be prepared for a very large fee.
This answer does not, nor is it intended to, create an attorney-client relationship or constitute attorney advertising. Rather, it is offered solely for information purposes. Since the facts of each case are different, it is critical to consult with qualified counsel with whom information can be shared and assessed under an attorney-client privilege, so that competent advice can be obtained on which you can make informed decisions.