PHP mail() won't work with IIS?

Couldn't get PHP mail() to work.  My problem was this:

phpinfo() showed that the SMTP parameter was set to 'localhost' in php.ini, which I thought should have worked.

Changing this to the explicit IP of the SMTP got mail() working!  I don't have direct access to php.ini, so at the top of my email function I added:

ini_set('SMTP', '10.xx.xx.xx'); // -where 10.xx.xx.xx represents the correct explicit value.