To route messages to external SMTP 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 asterisk (*) wildcard character. The * character indicates that the Send connector can be used to relay messages to all external 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 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, or an Edge server subscribed to an Active Directory site. 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 2010 source transport server you specify must be in the same Active Directory site.

For more information about how to configure your Exchange organization to send and receive Internet e-mail, see Managing Message Routing.

When Exchange is processing a message sent 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.

To learn more about Send connectors, see Understanding Send Connectors.

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You can disable and enable any connector in your organization. If a connector is disabled, it's not considered when routing messages. For example, if an Exchange 2010 Send connector is disabled, messages aren't routed to that connector.

Note:
Exchange Server 2003 doesn't detect the disabled state of connectors. Therefore, if Exchange 2003 is deployed in the same organization, it may route to that connector.

Routing only considers connectors in scope for the sending server. By default, no scope limitation is applied to connectors, and they are available to all Hub Transport servers in the organization. However, you can specify a local scope for a connector. If you configure a connector as scoped, the availability of that connector is limited to Hub Transport servers located in the same Active Directory site as the source servers for the connector. Scoped connectors aren't considered when routing by Hub Transport servers in other Active Directory sites.

The address space for a connector specifies the following:

• Recipient domains to which this connector routes e-mail
  
  
• Address space type
  
  
• Cost assigned to the address space for that connector
  
  

When you create a Send connector, the address space type is always configured as SMTP by default. Even though you can also specify a non-SMTP address space for a Send connector, the messages are still sent using SMTP. If you need to use a different transport protocol for transmitting messages to their destination, you must use a Delivery Agent connector or a Foreign connector.

When the Microsoft Exchange Transport service selects a connector for routing, it 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 connector for a particular domain, e-mail sent to that domain is always routed through that connector.

If more than one connector matches the address space for the destination recipient domain, the connector with the more precise address match is selected. 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 northamerica.contoso.com, e-mail addressed to user@europe.subdomain.contoso.com is routed to Send connector C1 and e-mail addressed to user@northamerica.contoso.com is routed to Send connector C2. Even though the latter address matches the address spaces on both connectors, the address space of C2 is a better match.

A message size restriction on a connector may also remove that connector from consideration during routing path selection if the message being routed is larger than the size restriction configured on that Send connector.

Connector cost is used to set selection priority when more than one connector is configured for the same address space. During routing, when the connector selection is made, the lowest cost routing path to the destination is selected. By adjusting connector costs, you can control the preferred routing path for mail flow in your organization and to the Internet. When you create a connector, the default cost is set to 1.

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In a coexistence scenario, the selection varies slightly depending on whether the source server for the selected connector is an Exchange 2010 or Exchange 2003 server.

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

1. Connector cost   The cost of the connector is the sum of the cost assigned to all the routing group connectors between the routing server and the routing group that contains the source servers for the connector and the cost assigned to the connector.
   
   
2. 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 they are spread across Exchange 2010 and Exchange 2003, Exchange 2010 will always prefer Exchange 2010 connectors.

When a connector is selected, if there are legacy servers listed as source servers as well as Exchange 2010 servers, messages are routed to the Exchange 2010 servers. When all things are equal, Exchange 2010 will always route the messages to other Exchange 2010 servers.

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 used for intra-organizational routing. For more information, see Internal Message Routing.

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.
  
  

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In this topology, a message is being routed from Active Directory Site A to the external recipient, john@subdomain.contoso.com. The following figure shows that two connectors can route messages to this address space.

The following table shows the configuration of two Send connectors in an Exchange 2010 topology.

Examples of Send connector configurations

In this scenario, the message is routed by using C2 because the most specific address space match is chosen. C1 has a lower routing cost, and the source servers are located in the same site as the Hub Transport server processing the message. However, the address space match takes priority over the other factors.

In this scenario, a message is being routed by a Hub Transport server located in Active Directory Site A to the external recipient, john@subdomain.contoso.com, as shown in the following figure.

The following assumptions apply:

• The routing server isn't listed as a source server for any Send connector.
  
  
• The IP site link between Site A and Site B is assigned a cost of 5.
  
  
• Two Send connectors can route messages to the address space. The following table shows the connector configuration.
  
  

Alternative Send connector configuration

Because both connectors have the same address space match, the connector cost is evaluated next. The cost assigned to connector C2 is added to the cost of the IP site link between Active Directory Site A and Site B, 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, john@contoso.com, as shown in the following figure.

The following assumptions apply:

• There are two Send connectors that match the destination address space equally.
  
  
• The source servers for the routing group connector between Exchange 2003 and Exchange 2010 are located in Site A.
  
  
• The routing group connector has a routing cost of 5.
  
  
• The IP site link between Site A and Site B is assigned a cost of 5.
  
  
• The source server for one of the Send connectors is an Exchange 2003 server located in routing group 1. The following table shows the connector configuration.
  
  

Connectors configured on different versions of Exchange

In this scenario, the aggregate cost of using connector C1 is 15, which is the sum of the IP site link cost and the Send connector cost. The aggregate cost of using connector C2 is 6, which is the sum of the routing group connector cost and the Send connector cost. However, even though C1 has a lower routing cost, Exchange will still route the message to the Exchange 2010 connector.

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