If all the users are in your Exchange organization, then Outlook tracks read reciepts automatically if the original sender requests one. Of course, the results are not reliable, since the receipt is generated when the user opens the message, but not when they view it in the preview pane. Some of the tools I suggested may have intraorganization variations in addition to their service options.
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Sue Mosher, Outlook MVP
Outlook and Exchange solutions at
http://www.slipstick.com
Author of
Microsoft Outlook Programming: Jumpstart
for Administrators, Power Users, and Developers