The Message Transport

The main purpose of a message handling system is to provide a means for transferring messages from one messaging server to another. This message transfer may occur on the same server, between Exchange Server 2003 servers in the same organization, between servers running Exchange Server and Internet hosts, or between servers running Exchange Server and foreign messaging systems. In all cases, the Exchange Server 2003 message transport engine provides the routing and relaying of e-mail messages.

Message Routing Architecture

In an Exchange Server 2003 organization, all messages are routed using SMTP. The SMTP protocol is also supported by all Internet messaging servers. If a server running Exchange Server sends a message to another messaging server that only supports the X.400 messaging protocol, the SMTP component in Exchange Server 2003 is responsible for routing the message. To accomplish this functionality, the SMTP component includes several subcomponents.

The following components are involved in every message transfer on a server running Exchange Server 2003:

• SMTP service   The SMTP service handles the SMTP communication between remote SMTP hosts and clients. This service implements the SMTP protocol commands that Exchange Server 2003 supports.

• Store driver   The store driver allows the SMTP service to communicate with the Exchange store to save messages that are passing through the SMTP service. The store driver also handles the delivery of messages for local recipients.

• Advanced queuing engine   The advanced queuing engine provides queue management and logic for message delivery, routing, and relaying.

• Categorizer   The categorizer provides categorization services for inbound and outbound messages. This component is also responsible for distribution list expansion using the LDAP and Active Directory.

• Routing engine   The routing engine provides the routing logic necessary to pass outbound messages to the correct messaging connector or SMTP virtual server.

• Queue manager   The queue manager controls link queues. Link queues are used to store messages that are waiting for transfer to the next remote destination.

For detailed information on message routing architecture and the relationships between these components, see Message Routing Architecture.

Message Routing with Routing Groups

Exchange Server 2003 uses routing groups to manage message delivery within an Exchange organization. A routing group is a collection of servers running Exchange Server that are connected by permanent network links.

Message delivery within a single routing group has the following characteristics:

• All message delivery is point to point   Within a single routing group, messages are always delivered from the sender's server running Exchange Server directly to the recipient's server running Exchange Server. Messages are never routed among multiple servers.

• All message delivery between Exchange Server 2003 servers uses SMTP   Exchange Server 2003 servers will always use SMTP to deliver messages to other Exchange Server 2003 servers in the same routing group.

• Messages are delivered as soon as the messages are received   Message delivery within a single routing group cannot be scheduled. If one of the servers running Exchange Server in a routing group is offline, the other servers running Exchange Server in the routing group will queue all messages to be sent to that offline server.

• Message delivery is automatically configured between Exchange Server 2003 servers in the same routing group   There is no way for an administrator to modify the message flow within a single routing group.

Message delivery within a single routing group is illustrated in the following figure.

Message routing in single routing group

Exchange Server 2003 supports the use of multiple routing groups. Message delivery between routing groups has the following characteristics:

• Routing group connectors must be configured between the routing groups.

• All messages sent between routing groups flow through bridgehead servers in each routing group.

• Message delivery between routing groups can be configured. Configuration options include scheduling message delivery, limiting message size, and restricting users or groups from sending messages across the connector.

The following figure illustrates message routing between multiple routing groups.

Message routing between multiple routing groups

Exchange Server 2003 supports the following three routing group connectors:

• Routing group connector   The routing group connector is the recommended connector for connecting routing groups that are in the same Exchange organization. This connector uses SMTP to transfer messages to other servers running Exchange Server 2003. The routing group connector can only be used to connect routing groups.

• SMTP connector   The SMTP connector establishes a messaging route between two routing groups or between a routing group and a non-Exchange SMTP host. Although the routing group connector and the SMTP connector use SMTP as the transport protocol, the SMTP connector provides additional functionality in that it can be used to connect an Exchange organization with any SMTP server.

• X.400 connector   The X.400 connector establishes an X.400 messaging route between two routing groups or between a routing group and an X.400 system. Like the routing group connector and the SMTP connector, an X.400 connector can be used to link Exchange routing groups. Generally, X.400 connectors are used only when connecting to other X.400 messaging systems.

Exchange Server 2003 supports the following optional connectors that you can use to connect the organization to non-Exchange messaging systems:

• Exchange Connector for Lotus Notes   Exchange Connector for Lotus Notes is used for message routing and directory synchronization between an Exchange organization and a Lotus Notes messaging system.

• Exchange Connector for Novell GroupWise   Exchange Connector for Novell GroupWise is used for message routing and directory synchronization between an Exchange organization and a Novell GroupWise messaging system.

• Exchange Calendar Connector   Exchange Calendar Connector is used for exchanging free/busy information between an Exchange organization and a Lotus Notes or Novell GroupWise messaging system.