Sorry - but you are an open relay - I just tested it:
220 yourserver.yourdomain.com Microsoft ESMTP MAIL Service ready at Wed, 21 Jul 2010 13:45:15 -0700
May be an open relay.
0 seconds - Good on Connection time
0.281 seconds - Good on Transaction time
OK - 173.13.xxx.xxx resolves to 173-13-xxx-xxx-abcd.xyz.comcastbusiness.net
Warning - Reverse DNS does not match SMTP Banner
Please have a read of my recent blog article and close your open relay down:
http://alanhardisty.wordpress.com/2010/07/12/how-to-close-an-open-relay-in-exchange-2007-2010/