Understanding SMTP Connectors


SMTP connectors are used primarily to connect to other mail systems or to define additional options for an SMTP Internet gateway. SMTP connectors can also be used to connect a routing group to another routing group internally, but an SMTP connector is generally not recommended for doing so. Essentially, SMTP connectors allow you to designate an isolated route for messages to flow either to a specific domain or over the Internet.

One advantage to using an SMTP connector is that you can specify additional configuration settings to affect mail delivery. These settings include:

  • Outbound mail delivery

    When you configure a connector, you can route mail in one of two ways:

    • Use DNS to route all outgoing mail through the connector. If you use DNS to route outgoing mail, the SMTP connector uses DNS to resolve the IP address of the remote SMTP server, and then it delivers the mail.

    • Specify a smart host (another server to which the connector routes all mail). The smart host takes responsibility for DNS resolution and delivers the mail.

  • Local bridgehead servers

    An SMTP virtual server hosts a connector. When you create a connector, you designate at least one Exchange server and SMTP virtual server as bridgehead servers. The connector inherits size restrictions and other settings from the SMTP virtual server; however, you can override these settings on the connector. You can also designate multiple bridgehead servers for load balancing, performance, and redundancy.

  • Address space

    The address space defines the mail addresses or domains for the e-mail messages that you want to route through a connector. For example, an address space of * (asterisk) encompasses all external domains—this connector is used to route all external e-mail. If you created a second connector with an address space of *.net, Exchange would route all mail sent to a domain with a .net extension through the second connector. This action occurs because Exchange selects the connector that has the most similar address space. This setting is configured on the Address tab of the SMTP connector's properties.

  • Scope

    You can select either an entire organization or a routing group for the connector's scope. The scope is also defined on the Address tab of the SMTP connector's properties.

  • Delivery restrictions

    You can restrict who can send mail through a connector. By default, mail is accepted from everyone. These settings are configured on the Delivery tab of the SMTP connector's properties.


    By default, you cannot restrict mail unless you change the registry key settings. If you chose to enable delivery restriction, be aware that restricting delivery is extremely processor-intensive and can negatively affect server performance. For more information about how to enable delivery restrictions, see How to Set Delivery Restrictions on the SMTP Connector.

  • Content restrictions

    You can specify what types of messages are delivered through a connector. These settings are configured on the Content Restrictions tab of the SMTP connector's properties.

  • Delivery options

    If you connect to a network service provider to retrieve your mail, you can configure a connector to run on a specified schedule and implement advanced queuing and dequeuing features. These settings are configured on the Delivery Options tab of the SMTP connector's properties.

  • SMTP communication

    You can control how the connector uses SMTP to communicate with other SMTP servers. Specifically, you can specify whether the connector uses SMTP or Extended Simple Mail Transfer Protocol (ESMTP) commands to initiate a conversation with another server and control the use of the ERTN and TURN commands (these commands are used to request that another SMTP server send any e-mail messages that it has). These settings are configured on the Advanced tab of the SMTP connector's properties.

  • Outbound security

    You can also ensure that any mail that flows through the connector is authenticated. This is useful if you want to establish a secure route for communicating with a partner company. With this setting, you can establish an authentication method and require TLS encryption.

    All of these settings are configured by using the Outbound Security button on the Advanced tab of the SMTP connector's properties.

Functions of an SMTP Connector

SMTP relies on DNS to determine the IP address of its next destination server. To send mail directly to an external mail server, an SMTP connector must use DNS to resolve external domain names. Alternatively, the connector can simply forward mail to a smart host that assumes responsibility for DNS name resolution and delivery.

After you set up an SMTP connector, as long as the destination address matches the address space that is configured on the SMTP connector, the servers no longer route the mail directly; instead, the servers route the mail through the SMTP connector. (These servers are called either gateway or bridgehead servers.)

To illustrate this point, assume that you want all external mail routed through a connector to a bridgehead server, which is the only server that communicates with the Internet. To configure this, create a connector on the bridgehead server with an address space of * (asterisk), which specifies all external domains. When e-mail is sent to an external domain, Exchange automatically routes it to this connector, rather than an SMTP virtual server sending the external mail directly. If you have more than one connector, Exchange first attempts to route mail through the connector that has the most similar address space (which is the most restrictive address space).


In a mixed-mode environment, if you have an Exchange Server version 5.5 Internet Mail Connector, Exchange Server 2003 treats this connector as a valid route. If you experience problems sending or receiving Internet e-mail messages, check the MTA queues on the Exchange Server 5.5 server and the X.400 queues on the Exchange Server 2003 server. Exchange Server 2003 uses the MTA to communicate with earlier versions of Exchange.

Uses for an SMTP Connector

Because of Exchange Server 2003 virtual server functionality, it is not necessary to create an SMTP connector to allow for mail flow, to connect it to other servers in an Exchange organization, or to connect it to the Internet. Furthermore, you do not need a connector if all of your Exchange Server 2003 servers connect to the Internet and successfully perform Domain Name System (DNS) lookups for Internet addresses.

However, although it is not essential for Internet mail delivery, the benefits of using an SMTP connector are that it:

  • Provides simplified administration.

  • Provides limited exposure to the Internet.

  • Establishes an isolated route for communicating with another domain or another mail system.

  • Routes mail to another mail system or relays mail to another domain.

  • Allows multiple bridgehead servers for load balancing.

  • Allows you to control how SMTP is used to communicate with other servers.

  • Permits scheduled connection times with customized settings.

The following sections provide detailed information about each of these benefits. For more information about SMTP connectors, see Microsoft Knowledge Base article 294736, "When to Create SMTP Connectors in Exchange 2000 and Later."

  • Simplify Administration of Mail Flow

    An SMTP connector provides more administrative control over how Internet mail flows out of your organization. You can use an SMTP connector, or a set of connectors, to limit the available routes for outgoing Internet mail. Also, because you need only check the SMTP queues and other configurations on a single server, using a single server as a bridgehead server simplifies troubleshooting.

  • Limit Internet Exposure

    One of the primary benefits of creating an SMTP connector is that you can route all inbound or outbound external SMTP mail through a particular server or set of bridgehead servers. By designating an isolated route for Internet mail that uses a connector, you limit your Exchange organization's exposure to the Internet.

    To use an SMTP connector to route Internet mail, specify one server or a set of servers as your gateway to the Internet, create an SMTP connector, and then designate those servers as the source bridgehead servers of the connector.

  • Isolate a Route for Communicating with Other Domains

    You can also use an SMTP connector to establish an isolated route for communicating with other domains. This approach can be useful when you want to use secure communications with a particular company.

    In previous versions of Exchange, you can configure settings per e-mail domain. Although these options are not available in Exchange Server 2003, you can create multiple SMTP connectors, set address spaces for these connectors, and then specify the settings that you want for those domains.

    For example, suppose you want to use SSL to secure all e-mail messages that are sent to the military, but you do not want to use SSL for other e-mail communications. To achieve this outcome, you need two SMTP connectors:

    • One with an address space ofSMTP:*.mil

    • One with an address space ofSMTP:*

    Because Exchange routes all mail through the connector that most closely matches the address space, all mail that is destined for the.mil domain initially tries to pass through the*.mil connector. You can specify that the *.mil connector send mail to only one server (a smart host), and that it use SSL and require authentication. Because routing considers *.mil and * as two separate destinations, if the *.mil connector is unavailable, mail queues until the connector becomes available. Mail does not reroute through the SMTP connector that uses the * address space.

  • Load Balance with Multiple Bridgehead Servers

    When you have a single connector that is hosted by multiple bridgehead servers, the servers using the connector randomly select the bridgehead server that they use, thereby load balancing requests across the bridgehead servers. The situation is different if you have multiple connectors with the same address space, each with a single bridgehead server. The servers that use these connectors use a method based on the server GUID to determine which of the available connectors they will use. The algorithm may not evenly distribute the server selections across the available connectors. So, to achieve load balancing, it is recommended that you use a single connector sourced to multiple bridgehead servers.

  • Use Specific SMTP or ESMTP Commands

    You can use a connector to control how your Exchange servers use SMTP to communicate with other servers. To initiate SMTP sessions, you can choose whether your server uses the ESMTP commands or SMTP commands, and you can control what type of commands your server issues.

    When you configure an SMTP connection, the following communication options are available:

    • Send or do not send server-side or client-side ETRN/TURN commands.

      TURN is an SMTP command that allows the client and server to switch roles and send mail in the reverse direction without having to establish a new connection. ETRN is an ESMTP command that is sent by an SMTP server to request that another server send any e-mail messages it has. You can use these commands if you depend on a network service provider to hold your mail for you and deliver it upon request.

    • Request ETRN/TURN from specific servers.

    • Send HELO (an SMTP command) instead of EHLO (an ESMTP command).

      HELO is an SMTP command that is sent by a client to identify itself, usually with a domain name; EHLO is an ESMTP command with which a server identifies its support for ESMTP commands.

  • Schedule and Customize Outbound Connections

    You can use a connector to open an outbound connection at specified times. This functionality is helpful if you use a network service provider to deliver your outbound mail, or if you have limited bandwidth and want to control when external mail is sent.

    You can also configure a connector to:

    • Allow high, normal, or low message priorities for a domain.

    • Allow system or non-system messages.

    • Use different delivery times for oversized messages.

    • Queue mail for remote triggered delivery.

    • Set specific delivery restrictions.