Exchange Server 2003 is a "closed relay" by default. This means that unless your usernames and passwords are compromised, an unauthenticated computer (SMTP host) cannot relay messages. It may be possible that the default SMTP relay restrictions were removed; in which case, you will want to re-enable them.
Microsoft details the procedure for configuring SMTP relay settings on Exchange Server 2003 on its website. All of your Exchange servers will be able to continue to send outbound email because they are authenticated computers.
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