xforward

[Cube]

I have a question which is not really about ispcp, so i post it in the Small Talk board.
I have installed ispcp on my new server (Debian Etch) and now trying to setup amavis. I want that locally originating mail (e.g. from root, my spamreports-script etc.) is not scanned by spamassassin. So i enabled the MYNETS policy in amavis. So i also need smtp_send_xforward_command=yes in postfix's master.conf, which is already set by ispcp. IMHO postfix should now send the xforward-command each time it connects to amavis, but it does not. I found out that it is sent only if I send a mail through webmail, but not if I send it from console or from outside and so these mails are scanned by spamassassin.
It's the first time i work with xforward, perhaps I have understood something wrong. Any ideas?