Electronic mail transport has been one of the most prominent uses of networking since networks were devised. Email started as a simple service that copied a file from one machine to another and appended it to the recipient's mailbox file. The concept remains the same, although an ever-growing net, with its complex routing requirements and its ever increasing load of messages, has made a more elaborate scheme necessary.
Various standards of mail exchange have been devised. Sites on the Internet adhere to one laid out in RFC 822, augmented by some RFCs that describe a machine-independent way of transferring just about anything, including graphics, sound files, and special characters sets, by email.[1] CCITT has defined another standard, X.400. It is still used in some large corporate and government environments, but is progressively being retired.
[1] Read RFC 1437 if you don't believe this statement!
Quite a number of mail transport programs have been implemented for Unix systems. One of the best known is sendmail, which was developed by Eric Allman at the University of California at Berkeley. Eric Allman now offers sendmail through a commercial venture, but the program remains free software. sendmail is supplied as the standard mail transfer agent (or MTA) in some Linux distributions. We describe sendmail configuration in Chapter 12.
sendmail supports a set of configuration files that have to be customized for your system. Apart from the information that is required to make the mail subsystem run (such as the local hostname), there are many parameters that may be tuned. sendmail's main configuration file is very hard to understand at first. It looks as if your cat has taken a nap on your keyboard with the Shift key pressed. Luckily, modern configuration techniques take away a lot of the head scratching.
When users retrieve mail on their personal systems, they need another protocol to use to contact the mail server. In Chapter 15 we discuss a powerful and increasingly popular type of server called IMAP.
In this chapter, we deal with what email is and what issues administrators have to deal with. Chapter 12 provides instructions on setting up sendmail for the first time. The information included should help smaller sites become operational, but there are several more options and you can spend many happy hours in front of your computer configuring the fanciest features.
For more information about issues specific to electronic mail on Linux, please refer to the Electronic Mail HOWTO by Guylhem Aznar. The source distribution of sendmail also contains extensive documentation that should answer most questions on setting it up.