a small job to be done regarding email facility

We would like to have a facility for our site visitors to talk to each other through emails. It will be a way for our users to network with each other through email. The project will need a registration page (that is moderated by us). Once the user has been approved, they can send a group email to all other users that are also registered. The emails are seen by everyone who is registered. When someone replies, the reply is also seen by everyone.