config/main.inc.php
// 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'] = 'localhost';
Define you host here. It is clearly described.
