In /etc/roundcube/main.inc.php you have:
// 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'] = 'tls://mail.server.com';
Is this your mail server?
You must edit this file to reflect your servers