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.
Write Short Messages in the Subject Only
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.