The same laws apply. Just as threatening someone with a paper letter is illegal, threatening someone with e-mail is also illegal. Many people get up to 100 messages a day, and unless you provide them with some context they'll have no idea what you're writing about. If the original message is long or addresses several topics, delete the text that isn't immediately relevant to your message. Resist the urge to point out every typo in a message.
Life is too short.
Keep this in mind when you think you've been insulted. If you feel you must respond, wait a while to calm down. For some reason, e-mail users tend to overreact and write really nasty letters which only beget more nasty letters, ad infinitum. If someone asks you to stop sending him or her messages, quit sending messages. The use of electronic mail and other network communications facilities to harass, offend, or annoy other users of the network is forbidden, as are chain letters.