HSMF is the POI Project's pure Java implementation of the Outlook MSG format.
At this time, it provides low-level read access to all of the file, along with a user-facing way to get at the common textual content of MSG files. to all
There is an example MSG textual renderer, which shows how to access the common parts such as sender, subject, message body and examples. This is in the HSMF examples area of SVN. You may also wish to look at the unit tests for more use guides.