Chapter 4: The SMTP Service

Overview

IIS 6 provides the Simple Mail Transport Protocol (SMTP) service. SMTP allows one computer to send messages to another; incoming messages are routed to a mailbox on the host, if the messages’ destination is the host server. If a message’s intended destination is not for a mailbox or user account on the host, the SMTP server forwards the message to the intended destination.

When the SMTP server is forwarding a message from a client, it contacts the next host prior to accepting delivery from the client. If the next host cannot be contacted or refuses acceptance for some reason, the SMTP server will not accept the message from the client and will send the client an error message indicating a non-delivery event. An SMTP server, by default, directly contacts the server hosting the domain to which the intended message is addressed. SMTP servers may be configured in a variety of ways to offload the resources of authenticating a client, maintaining connection state, and forwarding messages. In some cases, the next host may not be configured to accept any messages, because some administrators establish SMTP servers that are devoted to sending messages to remove the burden from the SMTP servers that are receiving messages from clients. Despite its name, SMTP is not always so simple.

Using the IIS 6 administration tools, the SMTP server may be configured to distribute messaging services across various hardware and network topographies. Message routing, encryption, and authentication configurations can be customized to accommodate the unique characteristics of a particular network and organization.

Given this overall architecture, the administrator will likely want to control the following:

• How a client authenticates to the SMTP server prior to sending messages

• How messages are transported to the SMTP server from a client

• How messages are transported from the SMTP server

• How messages are routed from the SMTP server

• SMTP configurations for a given SMTP domain