If you know this person, which I am assuming you do and their communication attempts are threatening your well being, you can and should contact the law enforcement before it is too late.
Short of that, to make tracing you harder if not totally impossible, download and setup "Tor Browser" (just Google the term). Set up a new email with your choice of the free webmail providers. Send your email and abandon the account. Although, for someone with abundant resources, this will be no deterrent and probably, not being able to trace you will make them even more angry and making the pursuit more "enjoyable" for the lack of a better term. And if you send an anonymous email, what good will this make for you, should the things go law enforcement way one day ?