I actually tried many times to setup a mail server on my debian machine but never could find a good tutorial. I gave up because I lack the time.
edit: I just tried again, for more than a couple of hours. First I had to guess that I had to install MDB2_Driver_mysql. Set up a DNS to match my /var/lib/roundcube and now I'm struggling to understand how I could sign-in since he didn't create any user for me.