can anyone please recommend me a good internal messaging system that works with email?

simply like forum private messaging system, but instead of having a dbase of the users, it will look and use the email address.

since all the email address is in the same domain, i doubt it would be a problem.

i need something that is fully functional like an emailling system, yet has its simplicity of the private messaging system.

thanks! :D

something like a forum to track out or instant messenging like msn/yim/icq/aim ?

like a forum would be best...with some flavour from a normal email. ie attachment, priority, folder, sorting, etc.

eg take domain.com
John wants to send a message to Bob
John goes to his site and sends a message to Bob
Bob receives it from his own subsite
etc...