By default, users with Gmail accounts at your domain can send mail to and receive mail from any other email address. In some cases, you want to restrict who your users can exchange mail with. For example, a school might want to allow its students to exchange mail with the faculty and other students, but not with people outside of the school.
The official updates feed from the Google Apps Team. You can subscribe to receive notice of any new Google Apps updates, by email or RSS Feed. Very handy for domain admins, trainers and tech savvy teachers.