Exchange Online Can Require More Licenses

Exchange Server clients are licensed via Client Access Licenses (CALs) and Exchange Online clients via User Subscription Licenses (SLs). While in many cases a customer migrating from on-premises to online will require an equivalent (or nearly equivalent) number of client licenses, that is not always the case. In particular, there are scenarios where organizations will require a greater number of Exchange Online User SLs than they would require Exchange Server CALs. For some organizations, this licensing quantity differential may be material and thus important to factor into cost estimates and licensing negotiations.

Shared and application mailboxes. Shared mailboxes that require advanced compliance features (such as in-place hold) require User SLs. This requirement will affect regulated organizations that need to preserve shared mailbox data as business records or for legal discovery. Also, Exchange Online mailboxes used to send e-mail by line-of-business applications or on-premises appliances require a User SL. In Exchange Server systems, shared and application mailboxes do not require their own CALs for features such as in-place hold, although the users or devices that access the mailboxes do.