Postfix works reasonably well 'out of the box' (just as installed). If you normally send your email to your ISP's SMTP server then configure the 'relayhost' parameter in /etc/postfix.main.cf to point to your ISP's mailserver.
If that isn't the problem, please provide more detail. Are you sending from a client on one machine to the postfix server? What does your mail log have in it?
My mail server will recive mail form it onw domian at frowarded mail for a other domain but the username will be the same.
also I need some pop only users.
The smtp server is only allowed to send mail if it is send by pine or webamail running localy on the server.
It may transfer it to my isp mail server (this so it will not cost me any bandwith).
user will need to collect there mail via either pop3 or imap.
But I must not be possible to relay mail via my server
Is short my smtp must be able to receive mail but not send it is the program is not running local.