Check the php config settings for the mail server settings etc, normally in PHP it requires just a few lines to send an email as there are specific functions for it. Its most likely the POP / SMTP server addresses are wrong in the config file.
If not they might be doing it a different way, i.e connecting directly to the mail server in question , via sockets etc, but i seriously doubt that.
You can print out the config file settings by creating a PHP file on the server which just contains this " phpinfo(); ?>." without quotes.
Also this is a very useful site, http://www.php.net/
I would not advise sending any source to anyone, as the company in question may not be exactly chuffed about it.....
Si