On a newly installed Exchange Server 2013 Cumulative Update 3 (CU3) server (not an upgrade from an earlier 2013 build), when you are setting up a hybrid deployment and you enter the product key to activate it, you will receive the following error message:
Invalid Product Key.
This issue occurs because of a regression in CU3 for Exchange 2013 which causes the product to be mistakenly recognized as invalid.
You can safely ignore this product activation error message. Until this issue is resolved in the near future, there are no adverse effects from leaving the server unlicensed. This is a known issue that is scheduled to be addressed in SP1.
If you already deployed the product key for the Hybrid Edition on a server and later upgraded the server to Exchange 2013 CU3, the server will remain licensed, and the license will be displayed as valid.