How to Set the Priority of a Message in Outlook 2013
Choose Low, Normal, or High. Click the Close button (or press Esc) to close the Properties dialog box.
Usually Importance is set to Normal, so you don't have to do anything. Putting a Low importance on your own messages seems silly, but you can also assign importance to messages received in your Inbox, to tell yourself which messages can be dealt with later, if at all.