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When you select a message in the message list, you can see the message in the Reading Pane and access many different types of file attachments. You can customize how the Reading Pane works and appears, or you can turn it off.
In addition to using the Reading Pane to help you quickly scan your messages, you can also preview the first few lines of messages in your message list, and you can break up long messages by splitting the message window.
For more information on how to use and configure the Reading Pane, please follow the link below:
Use and configure the Reading Pane to preview messages