[imp] Compose window blank after clicking send button

[imp@amusementcity.com]

I've got Horde 2.2 and IMP 3.2.
In compose window after I click send button blank window appears.
I tried to find sth in google but I couldn't find anything.
I think that it could be something with smtp but I don't really know what.
My /horde/config/horde.php look like this:

$conf['mailer']['type'] = 'smtp';
$conf['mailer']['params']['host'] = 'localhost';
$conf['mailer']['params']['port'] = '25';
$conf['mailer']['params']['auth'] = 'smtp';
$conf['mailer']['params']['username'] = 'username at domain.com';
$conf['mailer']['params']['password'] = 'password';

Thank you for help.

Sam