A few years ago, I improved a bit using the Apache POI to extract message contents from .msg files. I am sure that they have a simple swing explorer / viewer that you can use to study the structure in the format of a composite document, but I cannot find it right now.
I managed to extract most of the information that interested me, but in the end she wanted to create a version of the message in mime format and could not extract all the information I needed in a format that I could use.
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