it would be great to use probably POIFS to create .msg files from java MIME data structures. Others have been asking for this - http://forum.java.sun.com/thread.jspa?messageID=3752249� and some people at least appear to have been able to modify existing .msg files (http://article.gmane.org/gmane.comp.jakarta.poi.user/1008/match=outlook and http://article.gmane.org/gmane.comp.jakarta.poi.devel/2716/match=outlook+msg and http://article.gmane.org/gmane.comp.jakarta.poi.devel/9225/match=outlook+msg)
see also https://bugzilla.mozilla.org/show_bug.cgi?id=350819 for a corresponding Thunderbird RFE