Cite appropriate references. Give credit to others for their ideas. Avoid sharing commercial software. Avoid lobbying and advertising. Be cautious with confidences.
Mail is addressed to the person or persons that the originator intended to read the message.
Forwarding that message to others or including portions of it in responses to others is a violation of confidence between the originator and the original addressee. Be careful that messages you receive don't have other messages below the one you received that were not intended for others. Request permission from the originator before forwarding. In writing a message that you specifically do not want to be forwarded, you should state that in your message. Summarize in your follow up or reply.