uddeIM
uddeIM allows users of your community to send Private Messages to each other (note that uddeIM is not a webmail component nor an email client). uddeIM wants to be simple and easy-to-use but it offers all the features you need in a Private Messaging System (PMS).
uddeIM is available in 34 languages. It integrates seamless with Community Builder, Joomlaboard, AlphaUserPoints, Fireboard, Kunena, Jom...