external smtp in php.ini


this modern love
Is it possible to use an external smtp address in the php.ini? I tried to set it to my ISPs smtp. In my phpinfo(); it shows the smtp as being the one I set it to however it seems that the emails arent going through. Any ideas if this can even be done?
Actually new question.... does anyone know how to get the postfix and mail(); working? postfix is sending mail, I can use mail.app to send email from postfix and as my smtp. However, phow do i set the postfix path in php.ini? In the sendmail_path ?