Write short subject only

Sometimes, messages we send are very short ("Spare coffee in room 407" for example). They are so short that they fit in an email's Subject header perfectly.

While there is no reason to repeat the very message in the body of the mail, you should tell the recipient that the subject is already the message. Most email clients display a summary of mailboxes containing at least the sender and — the subject.

To convey to the recipient that they need not open the message, you should
* put "(EOM)" (End Of Message) at the end of the Subject line.

You can also use "(SIM)" (Subject Is Message) or any other convention you coin, but EOM is the most commonly understood indicator.