PHP Mail 501 error

Hello, We are running MediaWiki on PHP 5.2.6 with Apache for Windows. Every time we try to use an IIS mail server as an MTA we receive a 501 error. When we use a Unix postfix based MTA it works fine. Could anyone please tell me what I need to do to modify my PHP.ini to get this to work? We have tried the workaround for Exchange 2003 as described in this article by Microsoft: http://support.microsoft.com/?id=291828 But this fixes a 501 in the HELO. The problem here is the rcpt to. It comes up as the following: 2008-06-23 17:39:58 10.1.100.80 xxx SMTPSVC1 xxxx xxxxxxxx 0 RCPT - +TO:<DanMashal+<dmashal@intertrust.com>> 501 0 27 44 0 SMTP - - - - Could anyone please provide me with some much needed insight on this problem? I have been searching the web and have found a few workarounds but none that actually work. Thanks, Dan