After you've created a new message, completing the To and Subject fields and typing the message text, click the Importance: High button on the Message window's toolbar to mark the message as high priority.
Click the Importance: Low button to mark the message as low priority. (You won't notice anything different about the message itself, but the selected button is highlighted in the Message window.)
Because a person can receive literally hundreds of messages a day, you may want to assign a priority to help the recipient see at a glance the importance of your message. For example, if your message requires immediate attention, you may want to mark a message as important. For messages containing jokes or other low-priority information, on the other hand, you may want to mark the importance as low. When the message is received, the recipient can view its priority in his or her Inbox. (If you assign no priority to an e-mail message, the message will include no importance indicators.)
Use the high-importance label sparingly. It's like the old fable about the boy who cried wolf: If you label all your messages as important, your sender may ignore messages that really are important.