User ID Stolen by spammer

I have a couple of users who are getting mail returned to their mailboxes regarding e-mail they have never sent. These are spam messages that are being sent out under their e-mail accounts without their consent or knowledge. Is there any way to stopthis from occuring. We are using Exchange 5.5 and Outlook 97 and 2000.

About 5 months ago we closed our Exchange Server to Open Relays. However just lately spammers are starting to send out e-mail with our users return e-mail addresses. I had posted an earlier message and accidently awarded the points because I am new to the Q&A thing.