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','[email protected]');
ini_set('SMTP','working.server.com');
mail('[email protected]', '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?