Law enforcement agencies estimate that electronic communications are a factor in from 20 percent to 40 percent of all stalking cases. If you are being harassed, or even stalked, via email, you have a couple of options, particularly if you know who the person sending you the emails is. First, contact the senders ISP (Internet Service Provider) and make a complaint, which could result in this person losing their email account, and, at any rate, sets up a trail of evidence. Second, keep all the emails sent to you as further evidence. Third, do not reply to the harasser. Fourth, if there are threats of violence, or of a sexual nature, you should contact your local law enforcement agency immediately, because this can constitute a crime.
Many areas do not have specific laws that cover email harassment, or cyberstalking. However, in most areas, previously existing laws covering stalking are often applicable to cyberstalking. You can get a complete list of cyberstalking laws, or at least state laws that apply to email harassment at the National Conference of State Legislators.