Hi Bill,
You can see the answer here:
Exchange Server 2007 Client Access Licenses
Exchange Server 2007 is offered in two client access license (CAL) editions: Standard CAL and Enterprise CAL.
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The Exchange Server Standard CAL provides access to e-mail, shared calendaring and Outlook Web Access (OWA). In addition you will get advancements that reduce the cost and complexity of the messaging system by giving IT Administrators the messaging protection their company demands, the anywhere access their end users want and the reliability they need.
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The Exchange Server Enterprise CAL is an additive CAL and requires that a Standard CAL is also purchased for each user or device. The Exchange Server Enterprise CAL provides access to Unified Messaging and advanced compliance, as well as Forefront Security for Exchange Server and Exchange Hosted Filtering for onsite and hosted antivirus and anti-spam protection.
http://www.microsoft.com/exchange/evaluation/editions.mspx
Regards,
Nuno