With Exchange Server 2007 edition, a new term called "Server Role" was introduced by Microsoft. It is a logical concept that is designed in order to utilize and organize Exchange features across one or more Servers. While in Exchange 2003, there were only two roles played by the Server: BackEnd and FrontEnd. In this segment, we will have an overview of the new Server roles for Exchange 2007, 2010, and 2013.
This is the backend Server that is responsible to host the mailboxes and the public folder database along with address list, room/equipment scheduling etc. Mailbox Server has straight support with MAPI clients and consumes highest amount of resources (RAM, CPU) of the set up. For email send and receive, an instance of Hub Transport should be installed with this Server role. The mailbox role (MR) can be configured simultaneously with Hub Transport, Client Access, and Unified Messaging Server role.
A mailbox Server can hold a limited number of storage groups per Server (for example 50 in case of Exchange 2007) where each storage group has its own set of transaction log. For high availability of database on mailbox Servers, replication Services can be adopted that includes Single-Copy Clustering (SCC) (where an active and passive copy of DB is maintained on different Servers), Continuous Copy Clustering (CCR), and some smaller enterprise adopt Local Continuous Replication (LCR).
This Server is responsible for delivering messages. Even of an email is being sent to same mailbox database; it will first pass through hub transport (HT) Server. An Active Directory with MR Server should also have HT role installed in same site. A hub transport Server is allowed to deliver messages out of an organization like to anti-spam system or to Edge Transport Server or to HT Server that on different AD site. A Hub Transport Server handles following responsibilities:
The Hub-Transporter can be installed in conjugation with Client Access, Unified Messaging, and Mailbox roles. Nevertheless, for load balancing and failover, many organizations prefer to install mailbox and hub transport role separately.
MS Outlook can directly connect to Mailbox Servers for accessing the user-email account and public folders. Non- MAPI clients like IMAP, POP3, web based clients etc connect to Exchange Server through Client Access Server. We can say the CAS is like a front end Server that was used in previous versions of Exchange®. However, the only different we could notice here is rather than proxying the requests, the Server process the requests directly.
Together with proving Exchange database access to non-MAPI clients, CAS provides features like Auto-discover where a user do not need to provide Server name and mailbox information at the time of configuring the profile. Installing Client Access Server role is not important but it proves helpful in sites where there is no direct access to internet. Meanwhile, CAS should be a part of Active Directory domains and should be a part of organization's network.
This is an optional Server role and is designed for DMZ (Demilitarized Zone) LAN network. The main job of this Server role is to control the SMTP gateway for emails that enter or leave an organization. When an unbound mail is being received by Edge Transport Server, it first scans it, and take necessary actions if the mail if found to be virus-infected or seems to be a spam message. So, setting up a HT Server role ensures that a clean message is delivered to the recipient within Exchange environment.
However, this Server role cannot be installed with any other role on the Server. Its installation has to be done on separate Sever which ascertains internal security of the network. Moreover, what makes this role different from other Server roles is it is not a part of Active Directory.
This Exchange Server role allows users merging the e-mail and voice messaging together. Unified messaging gives a platform for mailboxes to receive emails and voice messages in the inbox so that it is easy for the users to access them and administrators to backup. This helps the IT admins to manage their data networks and the telephony networks together. Once Unified Messaging Servers are deployed on a network, the messages can be accessed via OWA, telephone or mobile device, MS Outlook®.