Never send anything that you would not want to become public knowledge. At present it is possible to forge e-mail. You should not necessarily assume that a message is valid. Respect the copyright on material that you reproduce. Cite all references, quotes and sources used.
Plagiarism applies to e-mail messages as much as other documents.
Do not send threatening, abusive or bigoted messages. Offensive or harassing behaviour is prohibited and disciplinary or legal action may be taken against you. Do not give another person access to your e-mail account. If you are forwarding a message you've received, do not change the wording. If the message was a personal message to you and you are re-posting to a group, you should ask permission first.