I had similar problems with Windows running PHP 3.0.14 and trying to use mail(); to send email through the SMTP Service.
I was using NT 4 and IIS 5 and even though I set the smtp server correctly in the php3.ini file I got a Server Error message from the php script when it tried to connect with mail(); to send.
One trick I thought might have worked was to use Blat mail (do a search for blat mail you can even use my search portal at http://askpete.net if you like) like I have done before with Perl scripts running on Windows.
The other possibility might have been to have the script write the email message out to a file using fwrite(); (see php manual function reference at http://php.net) in the SMTP mail servers pickup folder.
Also The SMTP Service was accessible from the command line so I could have used exec(); (see php manual function reference at http://php.net) to send the email.
These alternative ways though would require your script to build a correctly formatted email message in a variable string before either writing it to a file or passing it as an argument to a command line program. So it will require a little more work than sending email through mail();
Either way I never got around to working through which solution was better since I did not have the time or the real need for it.
Hope this info helps you out.