A. No. There are two versions of Exchange server, but these relate to the advanced features available in the various versions, primarily to the number of supported databases on the server. You can have a user with a standard CAL communicating with an Enterprise Exchange server. You can also have a mix of Standard and Enterprise CALs.

The Enterprise version of the CAL adds specific capabilities:

  • Unified Messaging with Voicemail integration.
  • Integrated archiving.
  • Advanced Mobile Management Policies such as configuration of Bluetooth, browser use, camera, personal email, Internet sharing, IrDA, Remote Desktop, Wi-Fi, application settings (signed and approved) and desktop synchronization. See this TechNet site for a full list.
  • Journaling at a user or distribution list level instead of per database.
  • Custom retention policies and managed folders.
  • Multi-Mailbox search and legal hold.
  • Forefront Security for Exchange Server and Forefront Online Security for Exchange.

Microsoft's site has detailed information on Standard vs. Enterprise Exchange editions.