This plugin is not needed for versions >= 1.5 of roundcube, as the functionality was added by the developers!
Roundcube allows to access multiple IMAP Server, but all mails have to be send over the same SMTP server. This plugin allows to add multiple SMTP Server (one per IMAP Server) to Roundcube.
// normal declaration of multiple IMAP Server
$config['default_host'] = array(
'ssl://imap.example.de' => '.de',