Exchange 2013 has brought major architecture changes and now there are just two roles – Client Access Server (CAS) role and Mailbox Server role, whereas the previous version of Exchange had five server roles.
Mailbox Server role: It includes traditional features like Mailbox Database, transaction logs, but now it also hosts Client Access protocol, Transport service and Unified messaging server.
Client Access Server role: It is an office client access protocol like HTTP, POP and IMAP and SMTP. Outlook no longer uses RPC to connect to the Client Access Server, it uses RPC over HTTPS (also known as Outlook Anywhere) and Outlook client doesn’t connect using fully qualified domain name (FQDN) or CAS Array as it was done in all previous versions of Exchange. It uses user’s mailbox GUID + @ + the domain portion of the user’s primary SMTP address to connect to the mailbox. This is a huge benefit to overcome limitations and complexity of Exchange 2010 RPC Client Access service on the Client Access Server.
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