DNS question for a brand new 2003 server running exchange.

I just setup a new windows 2003 server. I installed the basic server plus IIS and exchange. It is running on a dsl line with a static ip through a linksys router. I setup port forwarding for all of the ports I am going to need. When I login to godaddy the first thing I did was setup domain forwarding through their wizard. I set the domain to point to the static ip. This worked great and when you type in the domain the website comes up. Next I loaded up their mx wizard. the only options it gives me are to add a mx record and then specify a server for it. I put in the static ip of the server and set it as the first mx record. Now when I send mail to one of the accounts I created I get this error - PERM_FAILURE: SMTP Error (state 10): 550 5.7.1 Unable to relay

I can login to exchange from the webclient and send mail but I cannot receive mail. I have never setup dns before and when I look at the dns settings on the server the records point to 192.168.0.157 which is the local static ip I have setup that the router forwards to. I assume that the dns is correct because the website loads fine its just the mail that doesnt work. I can enter an advanced mode on the godaddy page for setting my DNS where I can change all the records. If someone can help me out I would be much appreciative.