Before you can read, delete, print, or otherwise manipulate a mail message, you must first select it. One message is already selected when you open your mailbox. The selected message has a rectangle (usually black) around the name, date, and subject of the message. That rectangle functions as a selection bar.
To select a different message in the list, click on the message, or use the up and down arrows to move to it. To select multiple messages, press and hold the Shift key and click on the messages. A check mark appears to the left of each selected message.