In the main.inc.php file if you look at username_domain you can map out and mail_domain you can define what domain to tack on to the username based upon what mail server they logged in on.

If you only allow one mail server (mail.yourdomain.com) to be used, then you can set the two items above like so:
$rcmail_config['username_domain'] = 'yourdomain.com';
$rcmail_config['mail_domain'] = 'yourdomain.com';

Otherwise, if you use multiple IMAP servers, you'd specify it like so:
$rcmail_config['username_domain'] = array('webmail.yourdomain.com'=>' yourdomain.com');
$rcmail_config['mail_domain'] = array('webmail.yourdomain.com'=>' yourdomain.com');

Hope that helps.

~Brett