Exchange Server 2013 RTM Cumulative Update 1
CU1 is the minimum version of Exchange 2013 required for on-premises coexistence with supported legacy
Exchange Server versions. The final build number for CU1 is 15.0.620.29. For more information on
coexistence, check out the Planning and Deployment documentation, and this Ignite webcast covering
deployment of and coexistence with Exchange Server 2013
Taken from the site of Thanks Team Blog
