Exchange Server 2003 as a Message Handling System

All messaging environments have several typical requirements in common. At a minimum, every messaging system in a messaging environment requires the following:

• A message transport mechanism

• A list of all users in the messaging system

• A place to store messages until the client retrieves them

• An interface that e-mail clients can use to communicate with a server in the messaging environment

General Components of a Message Handling System

The following figure illustrates the components of a message handling system.

Components of a message handling system

Exchange Server 2003 implements the following messaging components:

• Directory   The directory contains information about the users of the system. This information is used to deliver messages to the intended users. The directory also stores most of the configuration information about the message handling system. It includes information about how the system is configured and how to route messages from one messaging server to another. In Exchange Server 2003, Active Directory provides the directory. Many components in Exchange Server 2003 use a directory access module, known as DSAccess, to communicate with Active Directory. For more information about Exchange Server 2003 directory architecture, see Exchange Server 2003 and Active Directory.

• Message transfer subsystem   This component implements a routing and transfer mechanism for e-mail messages. The message may be intended for recipients on the same server or another server in the same organization, or for recipients on the Internet or other messaging systems. The central transport engine in Exchange Server 2003 is the Simple Mail Transfer Protocol (SMTP) transport engine, which is implemented in the SMTP service that originally is included with Windows Server 2003. Exchange Server 2003 extends the SMTP service to implement the message-handling features that Exchange Server 2003 requires. Be aware that message transfer in Exchange Server 2003 relies completely on the SMTP transport engine, even if sender and recipient reside on the same server. For more information about the SMTP transport engine, see SMTP Transport Architecture.

• **Message store **  In Exchange Server 2003, the message store (that is, the Exchange store) stores e-mail messages and other items in mailboxes and public folders. It also contains message tables that the transfer subsystem uses to store messages temporarily when messages are routed from one server to another. The Exchange store relies on Extensible Storage Engine (ESE) technology to implement the messaging databases. For more information about Exchange store architecture, see Exchange Information Store Service Architecture.

• User agent   The user agent provides access to the messaging system. In other words, the user agent is the messaging client. Exchange Server 2003 supports a wide variety of messaging clients, including MAPI clients, HTTP clients, and clients that use POP3, IMAP4, and Network News Transfer Protocol (NNTP). Lightweight Directory Access Protocol (LDAP) support for directory lookups, on the other hand, is provided by Active Directory.

The Message Handling System in the Network Infrastructure

To transfer a message from one server to another in an Exchange Server 2003 organization, the network must support TCP/IP. Both Active Directory and the SMTP service require TCP/IP. The following figure illustrates the components that are required for system communication and message transfer. You should be aware that specific components, such as Connector for Novell GroupWise, might require additional components that are not listed in this figure.

Networking components for Exchange Server 2003

Exchange Server 2003 requires the following networking components:

• IP and TCP   Exchange Server 2003 requires TCP/IP to communicate with other computers on the network. Exchange Server 2003 does not support other network protocols.

• DNS   Exchange Server 2003 requires DNS to resolve the IP addresses for other hosts on a TCP/IP network, locate domain controllers and global catalog servers in an Active Directory domain, and locate the e-mail servers in other messaging organizations.

• **DHCP and WINS **  Exchange Server 2003 does not require Dynamic Host Configuration Protocol (DHCP) to function. But some of the networking clients on the TCP/IP network may require this service. DHCP is used to automatically assign an IP address to computers on a network. Windows Internet Name Service (WINS), on the other hand, is used by Microsoft Windows clients to perform NetBIOS name resolution. In network environments that contain routers that do not forward broadcasts across network segments, WINS is required to resolve the IP addresses for other computers on the network. Exchange Server 2003 requires WINS.

• Windows Sockets   Exchange Server 2003 uses Windows Sockets to provide connection points for network clients connecting to services on a server. A Windows socket is a host's IP address combined with a port number that is used to identify a server service.

• Active Directory   Active Directory provides the directory service for Exchange Server 2003.

• Internet Information Services (IIS)   Exchange Server 2003 requires IIS to provide Internet protocol support. All the Internet protocol services, such as HTTP, SMTP, or IMAP4, run within the processing environment of IIS on the Exchange server. For more information about IIS, see Internet Information Services.

• Security Subsystem   Exchange Server 2003 uses a security subsystem of Windows Server 2003 to authenticate users in the Exchange organization. The security subsystem makes sure that only authorized users can access mailboxes or send e-mail to specified recipients.

• Windows I/O Manager   The I/O Manager on the server that is running Exchange Server manages the transfer of data between the server hard disks. Exchange Server 2003 uses the I/O Manager to access the transaction logs and databases that are stored on the server hard disk. The I/O Manager also controls the network file systems, such as server and client for Microsoft networks. Exchange Server 2003 shares several file-system folders for network access and accesses these folders using the Microsoft network file system.