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6. The Mail Hub and Delivery Agents

Contents:
The client.cf File
Define a Mail Delivery Agent
The local Delivery Agent
Add the Missing Parts to Mhub
Things to Try

Instead of having each individual workstation in a network handle its own mail, it can be advantageous to have a powerful central machine that handles all mail. Such a machine is called a mail hub and is analogous to the way Federal Express originally handled package delivery. In the old days, when you sent a package via Federal Express from, say, San Francisco to Paris, or even from San Francisco to Los Angeles, that package was first sent to Memphis, Tennessee (see Figure 6.1). The Memphis location was the Federal Express hub. All packages, no matter what their origin or destination, were sent to the hub first and then were transported outward from there.

Figure 6.1: The Federal Express hub approach

Figure 6.1

The advantage to this approach is that the Memphis hub was the only Federal Express office that needed the special knowledge of how to deliver packages anywhere in the world. [1] Local offices needed to know only how to deliver to the hub.

[1] Federal Express now has several regional hubs.

In a similar way, your workstations can be thought of as the outlying offices (client machines) and your central mail-handling machine as the hub. For mail, the hub performs the following functions for the entire site:

There are disadvantages to the hub approach when a site is composed of differing machine architectures and when machines are spread over many networks:

6.1 The client.cf File

The purpose of this tutorial is to develop a small sendmail.cf file for the clients of a hub scheme. As we did in the previous chapter, we will call our file client.cf. [3] We will be developing it in pieces and examining those pieces individually in the chapters that follow. In this chapter we will begin the process of developing a client.cf file from scratch. It will perform only two tasks:

[3] The client.cf file is a teaching tool. Although it can be used to send mail when it is finished, alternatives presented in Chapter 16, The null.mc File and m4 are generally more useful. Consequently, we never show the completed client.cf file.

In this chapter we tackle the first task: how to get mail from the client machine to the hub.


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