MS exchange mail function

Hi, I use ISS and PHP 4.2 . I can send an e-mail using mail function to intenal users our domain user but if I try to send an e-mail to outside of the our domain I got the following error message. Your message did not reach some or all of the intended recipients. Subject: Blah Blah Sent: 02.10.2002 13:40 The following recipient(s) could not be reached: soyukan@mail.koc.net on 02.10.2002 08:36 The recipient name is not recognized The MTS-ID of the original message is: c=US;a= ;p=TEI;l=PRIORIS0210020535TCPTQPC7 MSEXCH:IMS:TEI:MIS:PRIORIS 0 (000C05A6) Unknown Recipient $header = "From: cengiz@tei-tr.com"; $to = "soyukan@mail.koc.net"; /* if send to internal.user@tei-tr.com no problem*/ $message = "Test!"; $subject = "Blah Blah"; mail( $to, $subject, $message, $header ); Thanks.