Send your messages in plain text (do not use HTML). Limit your messages to no more than ~70 characters per line.
Unless you are using an encryption device (hardware or software), you should assume that mail on the Internet is not secure. Never put in a mail message anything you would not put on a postcard.
Do not include control characters or non-ASCII attachments in messages unless they are MIME attachments or unless your mailer encodes these. If you send encoded messages make sure the recipient can decode them.
Mail should have a subject heading which reflects the content of the message.
Be brief without being overly terse. When replying to a message, include enough original material to be understood but no more. It is extremely bad form to simply reply to a message by including all the previous message: edit out all the irrelevant material.
Do not send large amounts of unsolicited information to people.
If you are forwarding or re-posting 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.
If you think the importance of a message justifies it, immediately reply briefly to an e-mail message to let the sender know you got it, even if you will send a longer reply later.