postfix on ADSL

I was doing some research and am putting up a postfix mail server--when reading up I find that postfix does not allow relaying "except for the local subnet" just a though but as I am going to be nat'ing my ADSL connection doesn't this mean I am an open relay?? just a thought as if this is the case I (and whoever else might have missed it up till now) would need to adjust the postfix config to account for this. Thank You David Blomberg (sorry spammers its not up yet so get away from my mail server)