The cryptic, often-uninflected style of email writing can result in messages being interpreted in much harsher terms than the writer intended. Biting criticism and personal attacks are unfunny and unwanted. Remember, there is a real person on the other end. You are responsible for what you write. Keep your messages short to conserve bandwidth and save the resources of those subscribers who must pay to receive messages.
Some gateways also limit the size of mail they accept.
Use a descriptive subject line. Many people decide whether they want to read a message based on the content of the subject line. Sign all your messages. It helps to have at least a name to associate with a post and some mailers strip off the header, so the receiver cannot reply unless there is some kind of internal address. Be tolerant of newcomers.