This is easy:
In config/main.inc.php change the following setting:
// 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'] = '';
Between the single quotes, just put your mail server, whether it's mail.domain.com, or 127.0.0.1
Now when you go to the login page, there is no server box....
