Another major change in Exchange 2013 is in the role of Microsoft's RPC-based Messaging API (MAPI). Previously, the Outlook clients used two transport options for RPC traffic: HTTP or TCP. With Exchange 2013, RPC/TCP is no longer supported as a connectivity solution; only RPC/HTTP (also known as Outlook Anywhere) is supported.

Changing the protocols used for communication between the CAS and Mailbox server roles from RPC/TCP to a protocol that is more tolerant to latency and throughput over Internet and WAN connections allows these roles to not be as tied together from a geographical or user affinity perspective as they were before. This change allows CASs in one Active Directory site to authenticate and proxy requests to a Mailbox server located in a different site.

Exchange 2010 introduced the new shared RPC Client Access namespace that Outlook clients use to connect. Exchange 2013, instead of using an FQDN for the RPC endpoint, it uses a mailbox GUID together with...