Thanks alot Falko, using authentication for the relayhost works!
I was just wondering though, is this method supposed to make all email addresses relayable or only the one belonging to the authenticated user name and password? I noticed I can now relay my dyndns email address and the one I setup the authentication for but not the gmail address. My email I authenticate belongs to a different ISP from the one my adsl is subscribed to and I don't use my adsl ISP smtp. Can this be the reason?