Mail Problems

Hi everybody! My WebHost is using IIS on his server and has PHP 4.3.1 in CGI-Version installed (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 abusing the server for spaming. Did anybody experience similar problems? And if so, is there a workaround or a solution? Any help would be warmly appreciated, Matthias St.