There are many new criminal internet laws. Most are state laws, but there there may be federal laws as well if there is a crossing of state lines.
It would be in your best interest to make a police report at your local police department. Ask that it be referred to detectives for investigation. You need to bring a copy of the e-mail and the threat. The bureau may do an investigation, and if enough evidence is found for the detectives to proceed, they will submit it to the prosecuting attorney's or Attorney General
's office for prosecution under applicable state law.
If the bureau tells you that there is not enough evidence to proceed in Illinois, or that it would need to be prosecuted in Tennesee, then you need to contact the police jurisdiction where your lady friend lives, and also lawyer who specializes in criminal law and/or internet related crimes in Tennesee. You can contact the local bar association in the county where the lady that you are seeing lives and ask for a referral. It may give you 2 or 3 names. You can contact the attorney(s) by telephone for a consultation. An initial phone conversation may be free. You can go over the specific facts of the case with him/her and decide how to proceed.
You may also find it informative to contact the local bar association where you live for a referral as well, Besides a phone conversation, you may ask for an appointment. Again, sometimes these referrals are free. The attorney may give you additional guidance as to whether there are may be any federal law issues that may apply.
Internet crimes are relatively new. The facts that may allow prosecution are very specific. Unfortunately, you must make sure that your lady friend or other person didn't send the e-mail. Especially, in a criminal setting, it must be proven beyond a reasonable doubt that the boyfriend of your friend's mother is the one who actually comprised and sent the e-mail. That MAY be difficult, but certainly not impossible to prove.
Finally remember, just because an e-mail may be "deleted", it still remains inside of the computer, and may be retrieved, if necessary, by law enforcement.
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Legal Disclaimer: The answer provided herein is for informational purposes only and should not be considered legal advice. The information, research and opinions set forth above are limited to the facts presented in the question and no guarantee is made regarding the adequacy or accuracy of the answer. The answer provided does not infer or imply the establishment or acceptance of any attorney-client relationship. No claim is made that I am licensed to practice law in the state or jurisdiction where this information is being provided and you should always seek legal counsel admitted to practice in your local jurisdiction for representation and advice on any legal matter.