Understanding the SMTP Virtual Server

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SMTP virtual servers provide the Exchange mechanisms for managing SMTP. Each SMTP virtual server represents an instance of the SMTP service running on the Exchange server. You use Exchange System Manager to configure SMTP virtual servers that control the behavior of SMTP.

Essentially, an SMTP virtual server is an SMTP protocol stack (a process or server that both receives e-mail messages and acts as a client for sending e-mail messages). Each SMTP virtual server represents an instance of the SMTP service on a server. An SMTP virtual server is defined by a unique combination of an IP address and port number. The default SMTP virtual server uses all available IP addresses on the server and uses port 25 for inbound connections. A single physical server can host many virtual servers.

You use Exchange System Manager to control most of the SMTP settings. The property settings of the SMTP virtual server control inbound mail and, to a lesser degree, outbound mail settings.

Important

Because an SMTP virtual server plays a critical role in mail delivery, use caution when you modify its property settings. For example, the default SMTP virtual server sends messages within a routing group. Additionally, if the server is a domain controller, Active Directory uses this virtual server for SMTP directory replication. Therefore, instead of modifying the default SMTP virtual server, it is recommended that you either create an additional SMTP virtual server or create an SMTP connector to override the default virtual server settings.

Misconceptions about Multiple SMTP Virtual Servers

A common misunderstanding is that creating multiple SMTP virtual servers on a single Exchange server increases throughput. It is important to understand that each SMTP virtual server is multithreaded. Creating additional SMTP virtual servers on a single Exchange server does not increase performance and introduces complexity in your Exchange organization. An example of a case in which multiple SMTP virtual servers are required is a dual-homed server configuration. For most other scenarios, using the default SMTP virtual server with its default settings is generally sufficient.

Inbound Mail Settings on the SMTP Virtual Server

You can use the virtual server's property settings to configure the following inbound settings:

  • Inbound ports and IP addresses   The SMTP virtual server listens on its assigned IP address for incoming communications and accepts inbound connections on its assigned port. To configure these settings, use the Generaltab of the SMTP virtual server's properties.

    Important

    The SMTP service defines port 25 as its standard port. Do not change this setting.

    Note

    Upon installation in its initial configuration, the default virtual server connects to the remote SMTP server on port 25 to send outbound mail. This is a separate setting from the inbound port setting. To configure this setting, use the Outbound Connections button on the Delivery tab.

  • Relay restrictions   To prevent unauthorized users from using your server to send messages to external addresses, use the Relay button on the Access tab. By default, the default SMTP virtual server relays messages only for authenticated users.

  • Restrict submission and relay permissions to specific users and groups   In Exchange Server 2003, you can limit who can submit mail to an SMTP virtual server by using Relay and Authentication buttons on the Access tab.

  • Security   You can require Transport Layer Security (TLS), an implementation of Secure Sockets Layer (SSL), on incoming connections.

You can also configure other settings such as inbound connection restrictions, performance tuning, and handling of delivery reports notifications.

Outbound Mail Settings on the SMTP Virtual Server

If you want your SMTP virtual server to send mail directly to the Internet, you can configure outbound mail settings. Specifically, you can configure your virtual server to use an external DNS server to resolve external addresses and send mail directly to mail servers outside of your organization.

Important

Because an SMTP virtual server plays a critical role in mail delivery, use caution when modifying its property settings. For example, the default SMTP virtual server sends messages within a routing group. Additionally, if the server is a domain controller, Active Directory uses this virtual server for SMTP directory replication. Therefore, instead of modifying the default SMTP virtual server, it is recommended that you either create an additional SMTP virtual server or create an SMTP connector to override the default virtual server settings.

In many instances, it is preferable (but not required) to set up an SMTP connector to handle outbound mail. For more information about SMTP connectors, see Understanding SMTP Connectors.

Note

If you use an SMTP connector, it overrides some of the outbound mail settings and controls for outbound mail delivery.

To control outbound delivery on your virtual server, you can configure the following settings:

  • Outbound port

  • Outbound restrictions

  • Outbound delivery options

  • Outbound security

  • Performance tuning

  • Notification of delivery reports

For more information about how to configure these settings, see "Configuring Outbound Settings on SMTP Virtual Servers" in Connecting Exchange to the Internet.