romain,
I have not installed roundcube yet, but I ran across this:
$rcmail_config['mail_domain'] = '';
If you do not enter your domain into the above setting when users login to Roundcube they will be sending mail as "username@localhost" (where username is the user's login name). The user would need to manually edit their default Identity in Roundcube to the proper email address.
altered in /websites/roundcubemail/config/main.inc.php
I found this on page 4 of berylium's Guide to Installing Roundcube Webmail on Mac OS X Leopard
In this thread
I hope this works for you.
RedAntArmy