The piler gui can authenticate users against an IMAP server. To do so, set the following:

$config['ENABLE_IMAP_AUTH'] = 1;
$config['IMAP_HOST'] = 'mail.yourdomain.com';
$config['IMAP_PORT'] =  993;
$config['IMAP_SSL'] = true;

That's all, the gui will forward authentication requests to the IMAP server. If you need auditor or additional admin users, then create them manually in the gui as local users.