[otrs] Mail relay on windows

Hi, I'm having lots of trouble getting outbound e-mails to work on a windows setup. I have the system reading in mails from IMAP. But when I try to send an e-mail reply to a ticket I keep getting the error "553 The Sender is Spoofed" I'm trying to use the windows SMTP service through IIS. Is this error coming from IIS or is it coming from OTRS? Has anybody gotten this working before? Do I just need to tell OTRS not to check e-mail address syntax? Any help would be hugely appreciated. Rory Support my 365 Challenge in aid of the Irish Cancer Society www.365challenge.ie