Is my Exchange Server sending spam?

Three of the 12 users on an SBS 2003 server are receiving emails from themselves that are clearly spam with subject lines such as " Shall we meet after work", etc. The message body has a number of links to odd websites and various text, including information purporting to be from Microsoft. I have Kaspersky Workstation installed on all the users system plus Kaspersky for Exchange and Kaspersky for Windows Server on the server. Everyone has been scanned regularly and do not show any active problems. The Outlook Sent folder does not show the suspect emails but I find them in the Exchange Tools Message Tracking by searching by user over the previous two weeks, showing all of the emails sent by user. The search for emails received by user shows the spam emails as well. There are only a few of these spam emails, about 25 or so over a two week period, some days with none some days with half a dozen, sent 3-4 hours apart.

Our Exchange server is not on any blacklists, so far, and there doesn't seem to be floods of them going out to email addresses outside of the domain. They are only from each affected user to themselves. What could be causing this and  how can I get rid of it?