Relaying Prohibited Exchange 2003

If you haven't completely secured your Exchange server against relaying, an outsider can use your server to deliver email that appears to have originated from your server and possibly from one of your mail accounts. In the United States and other countries, people periodically debate whether systems' owners are liable if someone uses their system as a relay. The crux of the debate is whether an owner knew about the problem but chose to do nothing about it. Aside from concern about owners' liability, relaying is also a form of Denial of Service (DoS) attack because it takes system resources away from legitimate operations. Relaying is also bad for your company's business because it can generate ill will among your customers. 

If you're using Exchange Server 5.5, you have the tools to implement these recommendations and prevent the unauthorized use of your resources. To maximize your ability to use these capabilities and configure your system's antirelay defenses, you need to be running SP2 or later and the encapsulated relaying hotfix. An alternative to installing the hotfix is to install SP3, which includes the hotfix. In Exchange Server 5.5 before SP2, you could block relaying only by manually editing the Registry.