please use better thread titles in future, this is a help forum. the fact that you want help is a given. something like "help with sending mail via postifx" would be much more useful...
you need to provide more useful information about this setup. where are you trying to send mail to? is it a different domain or something? generally if this is a little box inside and AD infrastructure you'd use the smtp address for your exchange environmant as the default relay in postfix, .e.g "relayhost = my.exchange.server.com"