Licenses for Exchange Server are based on physical or virtual servers in combination with Client Access Licenses (CALs) for the users.
An Exchange Server license is required for every Exchange Server. This applies to both physical and virtual servers. The license is assigned to a physical server. Exchange may then run on the licensed server or in a virtual server on that licensed server.
For servers with a maximum of five mailbox databases, Exchange Server Standard is sufficient. Servers with more mailbox databases require Exchange Server Enterprise.
You may use an Exchange Server Enterprise license for a Standard installation. The other way around is not allowed.
Licenses are assigned to physical servers and are associated with them for a minimum of 90 days. If a virtual Exchange server moves across multiple physical hosts, the license must be able to move with it. Microsoft calls this License Mobility and that right is included in Software Assurance (SA). Exchange virtual servers without SA must remain on the same host, unless all hosts that can land Exchange virtual servers have sufficient Exchange Server licenses.
Client Access Licenses (CALs)
Licenses are also required on the user side. These can be licenses per user or licenses per end user device (PC, laptop, Thin Client ...).
A per user CAL (user CAL) allows a user to use an unlimited number of devices to access an unlimited number of Exchange Servers. With a CAL per device (device CAL), an unlimited number of users can access an unlimited number of Exchange Servers via the licensed device.
For most uses, an Exchange Standard CAL is sufficient. The Exchange Enterprise CAL is available as an additional CAL and offers the following additional functionality:
- In-Place Archive
- In-Place Holds (Indefinite, Query-based, and Time-based)
- Information Protection and Compliance
- Custom Retention Policies
- Per User/Distribution List Journaling
- Site Mailboxes – Compliance
- Data Loss Prevention
Exchange Standard CALs are part of the Core CAL Suite and the Enterprise CAL Suite. The Exchange Enterprise CAL is only part of the Enterprise CAL Suite. These suites are available on a per-user and per-device basis.
Exchange Online Plan 1 gives the same rights as the Exchange Standard CAL. Exchange Online Plan 2 gives the same rights as the Exchange Standard CAL plus the Exchange Enterprise CAL. These licenses are only available per user.
No CALs are required for external users. This applies to both the Standard and the Enterprise functionality.