New Exchange to has only 2 roles:client Access server and Mailbox server role.

Microsoft Exchange Server consists 2 server roles (reminds me of the Exchange 2000/2003 Front-end/back-end split)
To download the preview version:http://technet.microsoft.com/evalcenter/hh973395

Client Access server role. This role proxies connectivity-clients, such as Microsoft Office Outlook, Outlook Web App, mobile devices, POP, an D SMTP and also accepts mail from and delivers mail to other mail hosts on the Internet. Client access servers can organized into Client Access server arrays.

It includes the different components:the Client Access service and the Front End Transport service.

The Client Access service performs the following functions:provides a unified namespace, authentication, and net Work security. Handles all client requests for Exchange. Routes requests to the correct Mailbox server. Proxies or redirects client requests for legacy servers, such as Exchange-and exchange-client Access. Enables the use of Layer 4 (TCP affinity) routing.

The Front End Transport service This service runs on all Client Access servers and acts as a stateless proxy for All inbound and outbound external SMTP traffic for the Exchange, Preview organization. The Front End Transport service doesn ' t inspect message content, but it can filter messages based on connections, domains, senders, and recipients. The Front End Transport service is communicates with the Hub Transport service on a Mailbox server, and doesn ' t queue an Y messages locally.

Mailbox server role.   this role Stores mailbox data, performs processing and rendering for client connections proxied by the client Access server, and handles Unified Messaging requests. Mailbox servers can be organized to back-end clusters that use database availability groups (DAGS).

Mailbox Servers House The Mailbox data for the organization and perform data rendering and other operations. Mailbox servers can is grouped into back-end clusters which consist of database availability groups (DAG). Mailbox servers perform the following functions:host Mailbox databases. Provide email storage. Host public Folder databases. Calculate email address policies. Conduct Multi-mailbox searches. Provide high availability and site resiliency. Provide messaging records management and retention policies. Handle connectivity because clients don ' t connect directly to the Mailbox servers. Provide all core Exchange functionality for a given mailbox where that mailbox's database is currently activated. Fails over mailbox access is a database fails over.

Improvedfeatures in the Mailbox role for Exchange: Evolution of Exchange dag:transaction log code have been refactored for fast failover with deep checkpoint on passive Database copies. To support enhanced site resiliency, servers can is in different locations. Exchange Microsoft Preview now hosts some Client Access components, the Transport components, and the Unified Messaging Compone Nts. Exchange the Preview Store has been re-written in managed code to improve performance in additional IO reduction and Reli Ability. Each Exchange, Preview database now runs under its own process. Smart Search has replaced the Exchange multi-mailbox Search infrastructure.

Mail Flow

The transport pipeline consists of the following services:Front End Transport serviceThis service runs the all Client Access servers and acts as a stateless proxy for all inbound and outbound external SMTP TR Affic for the Exchange, Preview organization. The Front End Transport service doesn ' t inspect message content, but it can filter messages based on connections, domains, senders, and recipients. The Front End Transport service is communicates with the Hub Transport service on a Mailbox server, and doesn ' t queue an Y messages locally.Hub Transport ServiceThis service runs on all Mailbox servers and are virtually identical to the Hub Transport server role in previous versions of Exchange. The Hub Transport service handles all SMTP mail flow for the organization, performs message categorization, and performs m Essage content inspection. Unlike previous versions of Exchange, the Hub Transport service never communicates directly with mailbox databases. That task was now handled by the Mailbox Transport service. The Hub Transport service routes messages between the Mailbox Transport service, the hub Transport service, and the Front End Transport Service.Mailbox Transport ServiceThis service runs on all Mailbox servers and consists of the separate services:the Mailbox Transport submission Service A nd Mailbox Transport Delivery service. The Mailbox Transport Delivery service receives SMTP messages from the Hub Transport service, and connects to the Mailbox Database using the Exchange remote Procedure Call (RPC) to deliver the message. The Mailbox Transport Submission service connects to the Mailbox database using RPC to retrieve messages, and submits the Messages over SMTP to the Hub Transport service. The Mailbox Transport service doesn ' t queue any messages locally.

Quick Note:although not confirmed, the newest version of Microsoft Exchange, code-named "Exchange," was expected to Deb UT in the fourth quarter of 2012.