Exim: The Mail Transfer Agent

Chapter 3. Exim Overview

In the previous chapter, the job of an MTA is described in general terms. In this chapter, we explain how Exim is organized to do this job, and the overall way in which it operates. Then in the next chapter, we cover the basics of Exim administration before launching into more details about the configuration.

Exim Philosophy

Exim is designed for use on a network where most messages can be delivered at the first attempt. This is true for most of the time over a large part of the Internet. Measurements taken in the author’s environment (a British university) indicate that well over 90 percent of messages are delivered almost immediately under normal conditions. This means that there is no need for an elaborate centralized queuing mechanism through which all messages pass. When a message arrives, an immediate delivery attempt is likely to be successful; only for a small number of messages is it necessary to implement a holding and retrying mechanism.

Therefore, although it is possible to configure Exim otherwise, the normal action is to try an immediate delivery as soon as a message has been received. In many cases this is successful, and nothing more is needed to process the message. Nevertheless, some precautions must be taken to avoid system overload in times of stress. For example, if the system load rises above some threshold, or if there are a large number of simultaneous incoming SMTP connections, immediate delivery may be temporarily disabled. In these ...