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.Email started as a simple service that copied a file from one machine to another and appended it to therecipient's mailbox file.The concept remains the same, although an ever-growing net, with its complex routingrequirements 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.Quite a number of mail transport programs have been implemented for Unix systems.One of the best known issendmail, which was developed by Eric Allman at the University of California at Berkeley.Eric Allman nowoffers sendmail through a commercial venture, but the program remains free software.sendmail is supplied asthe standard mail agent in some Linux distributions.We describe sendmail configuration in Chapter 18.Linux also uses Exim, written by Philip Hazel of the University of Cambridge.We describe Eximconfiguration in Chapter 19.Compared to sendmail, Exim is rather young.For the vast bulk of sites with email requirements, theircapabilities are pretty close.Both Exim and sendmail support a set of configuration files that have to be customized for your system.Apartfrom the information that is required to make the mail subsystem run (such as the local hostname), there aremany parameters that may be tuned.sendmail 's main configuration file is very hard to understand at first.Itlooks as if your cat has taken a nap on your keyboard with the shift key pressed.Exim configuration files aremore structured and easier to understand than sendmail 's.Exim, however, does not provide direct support forUUCP and handles only domain addresses.Today that isn't as big a limitation as it once might have been; mostsites stay within Exim's limitations.However, for most sites, the work required in setting up either of them isroughly the same.In this chapter, we deal with what email is and what issues administrators have to deal with.Chapter 18 andChapter 19 provide instructions on setting up sendmail and Exim and for the first time.The includedinformation should help smaller sites become operational, but there are several more options and you canspend many happy hours in front of your computer configuring the fanciest features. Toward the end of this chapter we briefly cover setting up elm, a very common mail user agent on manyUnix-like systems, including Linux.For more information about issues specific to electronic mail on Linux, please refer to the Electronic MailHOWTO by Guylhem Aznar,[2] which is posted to comp.os.linux.answers regularly.The source distributionsof elm, Exim, and sendmail also contain extensive documentation that should answer most questions onsetting them up, and we provide references to this documentation in their respective chapters.If you needgeneral information on email, a number of RFCs deal with this topic.They are listed in the bibliography at theend of the book.What Is a Mail Message?A mail message generally consists of a message body, which is the text of the message, and specialadministrative data specifying recipients, transport medium, etc., like what you see when you look at a physicalletter's envelope.This administrative data falls into two categories.In the first category is any data that is specific to thetransport medium, like the address of sender and recipient.It is therefore called the envelope.It may betransformed by the transport software as the message is passed along.The second variety is any data necessary for handling the mail message, which is not particular to any transportmechanism, such as the message's subject line, a list of all recipients, and the date the message was sent.Inmany networks, it has become standard to prepend this data to the mail message, forming the so-called mailheader.It is offset from the mail body by an empty line.[3]Most mail transport software in the Unix world use a header format outlined in RFC-822.Its original purposewas to specify a standard for use on the ARPANET, but since it was designed to be independent from anyenvironment, it has been easily adapted to other networks, including many UUCP-based networks.RFC-822 is only the lowest common denominator, however.More recent standards have been conceived tocope with growing needs such as data encryption, international character set support, and MIME (MultipurposeInternet Mail Extensions, described in RFC-1341 and other RFCs).In all these standards, the header consists of several lines separated by an end-of-line sequence.A line is madeup of a field name, beginning in column one, and the field itself, offset by a colon and white space
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