It looks like you can't send private messages any more through the graph-api due to some abuse issues. If you're just trying to post to someone's news feed then you need to be sure that you request the appropriate permissions when generating your access token (either publish_stream or publish_actions depending on what exactly you're trying). The permissions are listed here:
https://developers.facebook.com/docs/reference/login/extended-permissions/
Details for requesting additional permissions in the android SDK are here:
https://developers.facebook.com/docs/android/login-with-facebook/#step3
Hope this helps.
Edit:
Re Email, there is no way to get a friend's email. The "email" permission gives you access to the user's email, but there is no equivalent for "friends_email". Facebook is pretty wary about giving apps the ability to privately contact people for fear of spam. Sorry.