php and SMTP

Hallo,
I'm newbie.
I have a mac pro with leopard server installed.

I would use it as webserver for some sites that I develop.

But.. I can't send mail from php.
I've tryed to start the "mail service" on server admin, but I don't know well how I have to configure it.

I've selected only SMTP and in the relay tab I've selected only my network.
But my script doesn't work.

Someone can help me?
Thanks!!

Mmh,
I think this is not the problem.
Before there was a server linux as web server (but I didn't configure it ).
And I didn't have problem.

I've set the firewall to consent SMTP traffic.

How can I see where is the problem?
In some log file?

Because my php app don't show me errors :S

I've found the problem!!
I haven't set well the name of domain so my pop server reject the mail!!

How can I be sure that my server is secure?
I've read something about open relay etc..

I accept (in the tab relay) only my ip address... is correct?

thanks