Question about mail()

Hi php-general, I have a question about the mail function. In the API I saw, that when running on linux, I just need the binary sendmail, usually located on /usr/sbin. Windows users need a running mail-server. I have php working on a linux machine. Why do I need to have postfix running then to make it work? I posted before, because no mails were sent. I had the sendmail-binary (postfix) installed, but postfix was not running. Maybe someone can explain it to me... SvT -- Who is the ennemy?