Well I don't know of a setting that would do that but you should be able to do it with a simple plugin. Here is a un-tested example of what the plugin should do:
<?php
/**
* Strip the domain from the username
*
* @license GNU GPLv3+
*/
class removedomain extends rcube_plugin
{
public $task = 'login';
function init()
{
$this->add_hook('authenticate', array($this, 'authenticate'));
}
function authenticate($args)
{
if (strpos($args['user'], '@') !== false) {
list($user, $domain) = explode('@', $args['user'], 2);
$args['user'] = $user;
}
return $args;
}
}