CAL (Client Access License) licenses for Exchange Server are essential for organizations using Exchange Server as a central platform for managing email, calendars, and contacts. These special licenses allow users and devices to legally access Exchange servers and services. Similar to other Microsoft products, CAL licenses for Exchange Server are available in two main variants: User CAL and Device CAL. A User CAL allows one user to access the Exchange server from any device, which is ideal for individuals who use multiple devices. On the other hand, a Device CAL enables access to the Exchange server from a specific device, regardless of the number of users using that device. The choice between User CAL and Device CAL depends on the specific needs and structure of the organization. Purchasing the appropriate CAL licenses is crucial for ensuring legal access to Exchange services and maintaining compliance with Microsoft licensing regulations.