Routing to External Domains

Microsoft Exchange Server 2007 will reach end of support on April 11, 2017. To stay supported, you will need to upgrade. For more information, see Resources to help you upgrade your Office 2007 servers and clients.

Applies to: Exchange Server 2007, Exchange Server 2007 SP1, Exchange Server 2007 SP2, Exchange Server 2007 SP3

This topic explains how Microsoft Exchange Server 2007 handles the routing of messages to external recipients. An external recipient is any message recipient that does not have a mailbox in the Exchange organization.

Send Connectors

To route messages to external domains, you must configure at least one Send connector to relay messages to the Internet. You can configure a Send connector and define the address space as the wildcard character (*). The * character indicates that the Send connector can be used to relay messages to all external Simple Mail Transfer Protocol (SMTP) addresses. You can also configure Send connectors to relay messages to specific address spaces when Send connector restrictions, such as message size, vary for those external domains.

When you configure a Send connector, you must select at least one source server for that Send connector. The source servers are the transport servers that are associated with that connector to handle message delivery. The source server for a Send connector can be a Hub Transport server, an Edge Transport server, an Edge Subscription, or an Exchange Server 2003 or Exchange 2000 Server bridgehead server. You can configure more than one source server for a Send connector to provide load-balancing and fault tolerance for the address spaces defined on that Send connector. However, each Exchange 2007 source transport server must have the same Active Directory directory service site membership. And each legacy bridgehead server must have the same routing group membership. For more information about load balancing for transport servers, see Load Balancing and Fault Tolerance for Transport Servers.

For more information about how to configure your Exchange organization to send and receive Internet e-mail, see How to Configure Connectors for Internet Mail Flow.

When a message is routed to an external recipient, the routing component of the Microsoft Exchange Transport service must select the best Send connector through which to route the message and then calculate the least cost routing path to reach that Send connector. The process that is used by routing to make this selection is discussed later in this topic. When the Send connector is chosen, you must consider several connector attributes. The following sections discuss Send connector attributes that affect routing path selection.

Send Connector Restrictions

Restrictions that are applied at the Send connector level may eliminate a Send connector from routing consideration. For example, if an Exchange 2007 Send connector is disabled, messages are not routed to that connector. However, if Exchange 2003 is deployed in the same organization, Exchange 2003 does not detect the disabled state and may route to that connector. A message size restriction on a Send connector may also remove that Send connector from consideration during routing path selection.

Connector Scope

Routing only considers connectors that are in scope for the sending server. By default, no scope limitation is applied to Send connectors, and they are available to all Hub Transport servers in the organization. However, an administrator can specify a local scope for a Send connector. If you configure a Send connector as scoped, the availability of the Send connector is limited to Hub Transport servers that are located in the same Active Directory site as the source servers for the Send connector. In Exchange 2003 and Exchange 2000, you can scope the availability of a connector to a routing group.

Address Space

The address space for a Send connector specifies the following:

• The recipient domains to which this connector will route e-mail

• The transport type

• The cost assigned to the address space for that connector

When you use the Exchange Management Console to create a new Send connector, the transport type is always configured as SMTP. To configure a Send connector that uses another transport type, such as Lotus Notes, use the Exchange Management Shell to specify the transport type as part of the address space. If you use any transport type other than SMTP, you must use a smart host to route e-mail. For more information about how to configure the address space by using the Exchange Management Shell, see Set-SendConnector.

Recipient Domains

During routing, a Send connector, to which e-mail is routed for delivery to the destination domain, is selected. The configuration of a Send connector defines one or more address spaces to which the connector will route messages. When it selects a Send connector, routing only considers connectors that have a matching address space for the destination domain. You can use the wildcard character * in an address space to indicate all domains, all domains that have a particular top level domain, such as *.com, or a second level domain and all its subdomains, such as *.contoso.com. When you configure a Send connector for a particular domain, e-mail that is sent to that domain is always routed through that connector. Also, the configuration settings for that connector are always applied to e-mail sent to that domain.

If more than one connector matches the address space for the destination recipient domain, the closest matching address space is selected. For example, if there is a single Send connector that is configured to have the address space *.contoso.com, Exchange routing will match both user@subdomain.contoso.com and user@contoso.com.com to the *.contoso.com address space and route to that Send connector.

If two Send connectors are configured to have overlapping address spaces, Exchange Server will route to the Send connector with the more precise address match. For example, if Send connector C1 is configured to have the address space *.contoso.com and Send connector C2 is configured to have the address space contoso.com, e-mail that is addressed to user@subdomain.contoso.com is routed to Send connector C1 and e-mail that is addressed to user@contoso.com is routed to Send connector C2.

Cost

Send connector cost is used to set selection priority when more than one Send connector is configured for the same address space. During routing, when the Send connector selection is made, the lowest cost routing path to the destination is selected. By adjusting Send connector costs, you can control the preferred routing path for mail flow in your organization and to the Internet.

When you create a Send connector, the default cost is set to 1. You can use the Set-SendConnector cmdlet in the Exchange Management Shell or the Send connector properties in the Exchange Management Console to modify the Send connector cost.

Foreign Connectors

Foreign connectors are used to send messages to third-party messaging systems. For more information about foreign connectors, see Foreign Connectors. If a message is being routed to an X.400 address space, Exchange 2007 must route to an X.400 connector that is defined on an Exchange 2003 or Exchange 2000 bridgehead server.

Selecting the Routing Path to an External Recipient

When a message is sent to an external recipient, Exchange 2007 must select a Send connector through which to route the message. Routing will always select a single connector through which to send the message. The selection varies slightly depending on whether the source server for the selected Send connector is an Exchange 2007 or Exchange 2003 server.

If more than one Send connector is configured to have an address space that meets the routing requirements for an external recipient, Exchange 2007 routing will select a single connector through which to route the message. The selected connector must meet the message size constraints. After Exchange 2007 has eliminated all connectors that have prohibitive message size restrictions, routing applies the following criteria to determine to which connector it will route:

• From the list of all Send connectors and foreign connectors that are configured in the Exchange organization, it narrows the list to connectors that satisfy all the following criteria:

  • In the scope for the local server

  • Enabled

  • With an address space that matches the recipient's e-mail domain

• From the resulting list, select the connector with the most specific address space match. No matching connectors may be found.

If more than one Send connector meets the address space match criteria, Exchange 2007 routing evaluates the following criteria to select a connector:

• Connector cost   The cost of the connector is the sum of the cost that is assigned to all the IP site links between the source Active Directory site and the Active Directory site that contains the source servers for the Send connector, and the cost that is assigned to the connector. The connector with the lowest aggregate cost is selected. If more than one connector has the same cost, the selection process continues to the next step.

• Proximity   The source server that has the closest proximity to the routing server is selected. This means that the local server is chosen over another Hub Transport server in the same Active Directory site, and a server in the local Active Directory site is chosen over a source server in a remote Active Directory site.

• Alphanumerically lower connector name   If more than one routing path has the same cost and proximity, the connector with the name that has the lowest alphanumeric value is selected.

If more than one connector meets the address space match criteria, and the connectors are hosted on servers that are running Exchange 2003 or Exchange 2000, the following selection method is used:

• Connector cost   The cost of the connector is the sum of the cost that is assigned to all the routing group connectors between the routing server and the routing group that contains the source servers for the Send connector and the cost that is assigned to the connector.

• Alphanumerically lower connector name   If more than one routing path has the same cost and proximity, the connector with the name that has the lowest alphanumeric value is selected.

The Exchange 2007 connector is always preferred by Exchange 2007 routing when the following conditions are true:

• More than one connector meets the address space match criteria.

• The source server of one connector is an Exchange 2007 server.

• The source server for the other connector is an Exchange 2003 or Exchange 2000 server.

After a connector is selected by using the previous criteria, there may be more than one routing path to reach the Active Directory site where the source server for the selected connector is located. In this case, the lowest cost routing path to the connector is calculated by using the logic that is used for intra-organizational routing. For more information, see Internal Message Routing.

The Send connector cost criterion is overridden when both an Exchange 2007 transport server and an Exchange 2003 or Exchange 2000 server are the source servers for a matching Send connector. Exchange 2003 and Exchange 2000 are not aware of the Exchange 2007 routing costs related to IP site links. Because Exchange 2003 and Exchange 2000 do not consider IP site link costs when calculating a least cost routing path, earlier versions of Exchange Server may calculate an Exchange 2007 connector as having a lower cost than an Exchange 2003 connector, even though the Exchange 2007 connector is assigned a greater cost. This can cause Exchange 2003 to route to a more costly Exchange 2007 connector. If Exchange 2007 always selected the least cost connector, regardless of the server version where the connector is sourced, Exchange 2007 could potentially route the message back to the Exchange 2003 connector. This can cause routing loops as messages are relayed back and forth between Exchange 2007 and Exchange 2003 routing groups. To prevent these routing loops, when a choice must be made between a connector sourced on Exchange 2007 and a connector sourced on Exchange 2003, Exchange 2007 always selects the Exchange 2007 connector, regardless of cost. This preference is only made when both connectors match the address space equally well for a recipient.

If no connector satisfies all criteria required to select a connector according to the logic described earlier, one of the following actions occurs:

• If there is no matching connector for an SMTP address space, the recipient is marked as unreachable and the message is routed to the Unreachable queue.

• If the message size exceeds the connector size restriction for all connectors, a non-delivery report (NDR) is returned to the sender.

• If there is no matching connector for a non-SMTP address space, an NDR is returned to the sender.

The following examples illustrate how messages are routed to external recipients. When a message is routed to an external recipient, the destination object is a Send connector that is configured to route messages to the recipient's SMTP address space.

The following table shows the configuration of two Send connectors in an Exchange 2007 topology. In this topology, a message is being routed from Active Directory Site A to the external recipient, Tjones@subdomain.contoso.com. Two connectors can route messages to this address space.

Examples of Send Connector configurations

In this scenario, the message is routed by using C2 because the most specific address space match is chosen.

The next example shows an alternative configuration for the Send connectors. In this topology, a message is being routed by a Hub Transport server that is located in Active Directory Site A to the external recipient, Tjones@subdomain.contoso.com. The routing server is not listed as a source server for any Send connector, and there is an IP site link that is assigned a cost of 5 configured between Site A and Site C. Two connectors can route messages to the address space. The following table shows the connector configuration.

Alternative Send connector configuration

The cost assigned to connector C2 is added to the cost of the IP site link between Active Directory Site A and Site C, for a combined cost of 15. The source servers for connector C1 are located in the local Active Directory site. Therefore, the IP site link cost to reach the connector is 0, for a total cost of 15. In this scenario, both connectors match the address space equally and have an equal cost. Routing selects connector C1 because it has a closer proximity.

In the next example, a message is being relayed from Active Directory Site A to the external recipient, tjones@contoso.com. There are two connectors that match the destination address space equally. However, the source server for one connector is an Exchange 2003 server. The following table shows the connector configuration.

Send Connectors configured on different versions of Exchange Server

In this scenario, the cost to reach each connector is computed as follows:

• To reach C1, the Hub Transport server in Site A must relay the message to Site B. Therefore, the cost to reach C1 is the cost assigned to the IP site link between Active Directory Site A and Site B (5) combined with the cost assigned to the address space on the Send connector (5+10=15).

• To reach C2, the Hub Transport server in Site A must relay the message to Site B to reach a source server for the routing group connector that provides a logical connection between Exchange 2003 and the Exchange 2007 routing group. The IP site link between Site A and Site B is assigned a cost of 5, and the routing group connector is also assigned a cost of 5. The cost to reach C2 is the cost assigned to the IP site link between Site A and Site B, combined with the cost assigned to the routing group connector and the cost assigned to the address space on the SMTP connector (5+5+1=11).

Although the cost to reach C2 is actually less than the cost to reach C1, Send connector C1 is selected as the routing destination because the source server is an Exchange 2007 server.

For More Information

For more information, see the following topics:

• Understanding Active Directory Site-Based Routing

• Internal Message Routing

• Message Routing in a Coexistence Environment

• Message Rerouting and the Unreachable Queue

• Load Balancing and Fault Tolerance for Transport Servers

• Send Connectors

• EdgeSync and Send Connectors