My experience is relaying is a BAD thing, not just for you, but the rest of the world as well. Generally, if you relay everything within your Exchange server and a very smart and wise hacker finds your server, you'll hardly know about the thousands and or millions of email messages that could be redirected through your server to the world and possibily put your organization in some very messing legal issues.
Oh ya ... there is an international organization that attempts to find sites that are relaying and sents you a very FIRM warning about that fact and do it to protect all of the rest of us.
David