Reverse E-mail Lookup Information HELP PLEASE!!!?

My parents received an e-mail from someone anonymous saying bad things about my girlfriend. My parents are believing this e-mail over me and my girlfriend is upset and I need to know how to get the name and that is it so I can stop this harassment. I can't use any pay sites and I just want a name and no other information. I have the e-mail address and I need a site that doesn't require payment.