It is impolite to forward someone's email messages to another person without the author's permission. Think before you post a message. A message written in anger or haste may be regretted later. Avoid flame wars. Angry responses to messages are called flames, and they usually provoke flames in response.
Flames wars are very annoying to other readers on a list or newsgroup.
Resist the temptation. If you inadvertently offend, apologize. Early email programs did not allow users to backspace in order to edit their work, and everyone understood that typing errors would occur. This is no longer the case, and errors now look like carelessness. When responding to a previous post, include the relevant parts of the original message.