It is legally considered harassment if someone sends you emails after you tell them to stop. You can block them and if they continue to send you e-mails under another name, you can bring them to court and sue.
If you know who the e-mails are coming from, and know how to contact them, then you can have a cease and desist order issued against them. If they violate the order, then they can be arrested.
Depending on the content, the author may be liable for misdemeanor or even felony harassment charges, or even menacing or threatening charges if the message threatens the recipient.
If the individual has actually threatened harm to you, your property, family members or your job, print it out and take it to the police to file a report.
