Internal mail system How do I allow users send emails to the other users?
What module can I use to allow users to email each other?
I don't know about 'best' (although @Berdir might have something to say on the matter ;) ), but Privatemsg would probably be perfect for this:


  Privatemsg allows your site's visitors to send private messages to each other.


It also provides e-mail notifications of new messages.