// the mail host chosen to perform the log-in
// leave blank to show a textbox at login, give a list of hosts
// to display a pulldown menu or set one host as string.
// To use SSL/TLS connection, enter hostname with prefix ssl:// or tls://
$rcmail_config['default_host'] = 'ssl://mx1.yourmailserver.com';
I think that you must add there the servers you want to access from roundcube. If you have more than one, a dropdown list will be displayed, then, you select the server and login.
As I could see, user/pass are validated with mail server databases, not with local.