if you do find it's an open relay, you're probably on a load of email blacklists. I advise putting in your IP at MXToolbox and contacting the list owners of any that you are on.
http://multirbl.valli.org/lookup/ is another good tool to check if you're on any blacklists, but you shouldn't request removal until after you've stopped relaying untrusted servers and fixed any other problems that got you on the blacklist.