I've installed PHP 5.2 with Web PI on Windows Server 2008. This server also houses the Exchange server that I use for mail.
The script I'm using uses PHP's mail() function to send mail.
When using the function, mail is delivered when the recipient is within the domain, however if recipient is outside, it fails to deliver and mail returns an error.
I can't seem to find anything in the log files, so I am assuming it is an Exchange 2010 error.