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Configuring the SMTP Service

Updated: August 22, 2005

Applies To: Windows Server 2003, Windows Server 2003 with SP1

The IIS SMTP service is a simple component for delivering and receiving e-mail messages. A message is transferred to a designated SMTP server to initiate the delivery. The originating SMTP server initiates communications with a DNS server, based on the domain name of the recipient e-mail address. The DNS server looks up and then returns the host name of the destination SMTP server for that domain.

Next, the originating SMTP server communicates with the destination SMTP server directly through TCP/IP port 25. If the user name of the recipient e-mail address matches one of the authorized user accounts on the destination SMTP server, the message is transferred to the destination SMTP sever, where it waits for the recipient to pick up the message through a client program.

The SMTP service also can transfer messages through one or more intermediate relay SMTP servers. A relay SMTP server receives the original message and then delivers it to the destination SMTP server or redirects it to another relay server. This process is repeated until the message is delivered or a designated time-out period passes.

The IIS SMTP service is commonly used to create a smart host, which is an SMTP server through which all outgoing messages are routed. For example, you can set up a smart host as a stand-alone SMTP server that sits between the Microsoft® Exchange server at your organization and the Internet. In this scenario, the smart host routes all e-mail from the Exchange server to the Internet and provides an added layer of protection between the Internet and the internal network. A smart host can also operate within a network.

Another common use for the SMTP service is to enable e-mail messages to be delivered from a Web site. For example, a company can provide an e-mail link for sending feedback messages or for requesting information. The SMTP service receives and routes those messages to the appropriate mailbox or to another SMTP server. Collaboration Data Objects (CDO) provides a simple object model that applications can leverage to submit mail to the SMTP server. For more information about CDO, see Overview of CDO on MSDN.

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