Cumulative Update 2 for Exchange Server 2013 resolves issues that were found in Exchange Server 2013 CU1 since the software was released. This update rollup is highly recommended for all Exchange Server 2013 customers.
MCSE: Exchange Server 2016, 2013
MCSE: Skype for Business Server 2015
MCSE: Azure Productivity
MCSE: Windows Server 2016
MCSA: Windows Server 2012, Office 365
MCITP: Exchange Server 2010-2007 | Lync Server 2010
VMware: vSphere 6 Foundations