[horde] Problem with _horde_hook_preauthenticate ... used also when imp autenticates?

[Giorgio Volpe]

I'm tring to use _horde_hook_preauthenticate as suggested in 
http://wiki.horde.org/ImapSelect to map horde username to real usernames ...
In no way I can see the function called when i try to log in!

I've activated the hook in horde3/config.php writing 
$conf['hooks']['preauthenticate'] = true;

I'm using imp as the authentication backend. Is 
_horde_hook_preauthenticate called only by horde authentication and not 
by imp?

Any int?

thanks  Giorgio