You can change the way in which your messages are displayed. You can change the message display in a message window, or in the preview pane.
To display full header information for your messages, choose View -> Message Display -> Show Full Headers. The header information contains technical details of the message, such as the server that delivered the message, the format of the message, and so on.
To display the full source for the message, choose View -> Message Display -> Show Email Source. The full source for the message is the source code for the message.
To return to normal display, choose View -> Message Display -> Normal Display.