Mail and NT4

I have recently installed PHP4 to accomodate some PHP development on an NT4, IIS server (SP6). When I try to run the mail() function from a simple script I can watch the server connect a session to the SMTP server, therefore proving that php.ini is corrrect however no mail is sent and the browser hangs until I either terminate the SMTP session or the browser times out. Also if I terminate the session from the SMTP server I get a 'failed to receive' error? Could anyone shed any light on mistakes I may be making. Many Thanks, Steve