How to integrate mailing system using codeigniter

[eluser]babai[/eluser]
I have a site softgen.co.in. So I want to integrate mailing system into my site. First member will register into my site by a unique username. Automatically will create a email id(Ex. [email protected]). After login into the system he can see any mail from other user and can reply the message. Also user can compose new message. just example of gmail, yahoo mail, etc. For this what I need? I want just simply mailing system. So please help me how will do this?