OS need anything for mail() to work?

I'm running PHP 5.2.5 on OS X 10.5.2, and using the following code: ini_set('sendmail_from','address@mydomain.com'); ini_set('SMTP','working.server.com'); mail('brian@mydomain.com', 'Subject', 'Message'); The servers and addresses are KNOWN TO WORK, I use this same code and same info on a number of different web servers around the world. Authentication is absolutely not the problem. This code works on other computers, but it doesn't work on mine. I get no error, but no email is ever sent. So my question: Is there something that needs to be installed, running, or otherwise configured on a computer to make sure PHP can send emails? How could I test for this?