To install postfix admin you need to setup postfix to handle virtual domains with MySQL.
On a debian system the setup.sh script should take care of that and a lot other stuff. You must edit the database and domain setting in this script first.
After that you must add the automatic reply domain to config.php:

$config['postfixadmin_autoreply_domain']='autoreply.intermeshdev.nl';
$config['postfixadmin_vmail_root']='/vmail/';

You might need to add this domain to /etc/hosts

The first part must be 'autoreply' and the latter must match the domain configured in setup.sh.

Now everything should work.