If it is a text message, you are already looking at the source of the
message in the preview pane (or when you double-click to view it alone).
If it is an HTML-formatted messages, right-click in the body of the
message and select to View Source. If you want to look at the headers
of a message, use View -> Options.
The PocketKnife Peek plug-in provides a toolbar button and a view window
to look at the headers and source. Might be more convenient for you.
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