Private Messaging plugin

hi all…
i’m doing a somehow networking site using wordpress and i want people who register on my wordpress site, to be able to contact directly (privately) to each other when they are logged in, probably through the dashboard etc.

how can i do this happen in practise?
is there any plugin that could help me?

P.S.. not a shoutbox or a public chatroom. I want/need something that will allow e.g. the member “A” of my site to contact the member “B” privately, as normal Private Messages (PM).

Thank you very very much for your attention