Exchange 2013 has simplified the Client Access Server (CAS) design architecture using global/single namespace. This is because of the way CAS works in Exchange 2013. Outlook no longer uses RPC protocol to access the email, but instead it connects to the CAS server and accesses the mailbox through https protocol. The CAS server makes a direct connection with the mailbox server, if the mailbox exists on the same AD site. If not, then it decides, whether to proxy the request or to redirect the request to another CAS/Mailbox infrastructure. It queries the Active Manager to determine the mailbox server hosting the active copy and it will proxy the request to mailbox server. This occurs irrespectively of a mailbox location. One CAS server redirects its request to another CAS server only if there is a telephony related request or an OWA request.
To read about more on this topic, please use the below link at blog.netwrix.com