Which are the requirements to enable Exchange Server 2016 Enterprise functionalities?

In order to use Exchange Server 2016 Enterprise functionalities, the user is required to be licensed with one Enterprise CAL (Client Access License) plus one Standard CAL.

Similar CAL combinations/requirements are also valid for Skype for Business as well as Sharepoint.

Source: Licensing overview_Microsoft_Exchange Server 2016_as is 2016-04-27