My Nabble Forum seems to be archiving the Google Group but when I try to post a message it doesn't seem to get through? I double-checked and both myself as well as the Nabble forum are members of the Google Group. Is there some kind of setting that could be the culprit here?
Maybe google has some kind of filter that prevents emails sent on behalf of other users from being accepted. We don't have any settings in this regard, so maybe you should investigate possible filters of google groups.