How to Create E-Mail Messages in Outlook 2013
Type the subject of the message in the Subject text box.
You should keep your subject line brief. A snappy, relevant subject line makes someone want to read your message; a long or weird subject line doesn’t.
If you forget to add a subject and try to send a message, Outlook opens a window that asks whether you really meant to send the message without a subject. Click the Don’t Send button to go back to the message and add a subject. If you want to send your message without a subject, just click the Send Anyway button (but not before you’ve written your message).