Suppressing X-Return-Path

Has anyone had experience with PHP or a mail server setting the X-Return-Path header on mail sent using PHP's mail() command? Our tests indicate the X-Return-Path is being set to the "sendmail_from" address found in php.ini. Our code definately isn't doing it, as a hello-world mail() example proved. The system is running Win 2K, all current service packs and hot fixes, IIS 5, php 4.2.3 (running as a CGI), and the Zend optimizer 2.0.1. The mail server is Microsoft Exhange Server. Does anyone know why the webserver or mail server would set the X-Return-Path, and if it can be suppressed? Regards, Darren Fehrmann Storefront.com