I have found a little solution if you are using roundcube for a lot of domains, the only "problem" is that you have to upload roundcube on all that domain.
Then, the config of your main.inc.php should be:
// 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 connection, enter ssl://hostname:993
$rcmail_config['default_host'] = '' . str_replace("http://www.", "", $_SERVER['HTTP_HOST']);
// TCP port used for IMAP connections
$rcmail_config['default_port'] = 143;
// Automatically add this domain to user names for login
// Only for IMAP servers that require full e-mail addresses for login
// Specify an array with 'host' => 'domain' values to support multiple hosts
$rcmail_config['username_domain'] = '' . str_replace("http://www.", "", $_SERVER['HTTP_HOST']);
Roundcube will automaticly detect the domail name and:
When a user want to login he just have to tape : "user", roundcube will add @domaindetected.com
And of course, no server to enter for the user, it will be: domaindetect.com
