mail()-problems with IIS

Hi there! My WebHost is using IIS on his server and has installed PHP 4.3.1 in CGI-Version (I dislike this solution). When I use mail() to send emails to local addresses everything works perfect. But when I try to send mails to extern addresses I get the following error: Warning : mail() [ function.mail ]: SMTP server response: 550 , Recipient unknown in D:\InetPub\wwwroot\abendstille\mail\test.php on line 2 The system-administrator says, that this error occours, because php changes the sender-ip to the ip of the user who acesses the page and that he configured the mail-server so that extern ips cannot send emails to extern addresses to avoid using the server for spaming. Is there a workaround, or did he forgot to set a value in php.ini? Any help would be warmly appreciated, Matthias St. www.abendstille.at