LPI Linux Certification in a Nutshell, 2nd Edition

Objective 2: Using Sendmail

First, it's important to understand Sendmail's function in regard to email. To do this, it is necessary to learn some terminology. Sendmail is a Message Transfer Agent (MTA)—the oldest one in existence. The MTA has two jobs: to ensure that mail messages are transferred between entities and to place those messages in a destination directory. The MTA can also authenticate parties, but this is just part of its function of transferring messages between users or hosts.

In contrast, a POP or IMAP server is called a Message Delivery Agent (MDA). The MDA's job is responsible for providing mechanisms that allow users to retrieve their messages. Both the MTA and MDA are usually server-based functions. The tool that you use to send and receive email is called a Message User Agent (MUA). The MUA is most often a client-side tool, though with web-based mail, the MUA's function becomes complicated to some extent, because the ability to send and receive email is more evenly split between the web browser and a server.

Sendmail is an old MTA; it has has been in constant use for decades. It is the subject of one of the largest O'Reilly books, the well-known "bat book," so called because it has a bat on its cover. This section in no way is meant to replace that remarkable achievement. However, this section will help you get a handle on how much you need to learn about Sendmail in order to pass the LPI Level 2 Exam. But before we move on with the specifics of Sendmail configuration ...