BuddyPress
Plugin – BP Messaging Control
BP Messaging Control is a plugin that offers very powerful controls over BuddyPress private and public messaging via a simple settings interface. The plugin maps out all the roles used on the site and allows each to be assigned No Messaging, Admin Only, Reply Only or Full Messaging. In addition email quotas can be assigned. This is ideal for monetizing or differentiating messaging as part of your membership site.