Exim: The Mail Transfer Agent

Chapter 4. Exim Operations Overview

The previous chapter used some fragments of a simple Exim configuration file to show how it goes about delivering a message. Later chapters go into more detail about the various options that can be used to set up configurations that can handle many different circumstances. However, if you have just installed Exim, or if you have inherited responsibility for an Exim system from somebody else, you most likely want to know a little bit about the basic operational aspects. This chapter is an introductory overview; the features that are described reappear later in more detailed discussions, and Chapter 21, covers Exim administration in more detail.

How Exim Identifies Messages

Each message that Exim handles is given a unique message ID when it is received. The ID is 16 characters long, and consists of three parts, separated by hyphens. For example:

11uNWX-0004fP-00

Each part is actually a number, encoded in base 62. The first is the time that the message started to be received, and the second is the ID of the process (the pid) that received the message. The third part is used to distinguish between messages that are received by the same process in the same second. It is almost always 00.

The uniqueness of Exim’s message IDs relies on the fact that Unix process IDs are used cyclically, so in practice there is no chance of the same process ID being reused within one second. For most installations, uniqueness is required only within a single host, and the ...