After you've double-clicked a message in the Inbox to open it, click the Next Item button on the message window's toolbar to display the next message.
The next message in the Inbox list is displayed. Click the Previous Item button to display the previous message.
The previous message in the Inbox list is displayed. Click the message window's Close (x) button to close the message and return to the Inbox.
Rather than return to the Inbox to open each message, you can automatically display the next (or previous) message in the Inbox list from within an open message window.
Displaying Other Messages
You can display other messages, including the last message in the Inbox folder, by clicking the down arrow next to the Next Item or Previous Item button and then clicking the desired item.