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 special characters, and the like. Much thought has also been given recently to ``multi-media mail'', which deals with including pictures and sound in mail messages. Another standard, X.400, has been defined by CCITT.
Quite a number of mail transport programs have been implemented for systems. One of the best-known is the University of Berkeley's sendmail, which is used on a number of platforms. The original author was Eric Allman, who is now actively working on the sendmail team again. There are two ports of sendmail-5.56c available, one of which will be described in chapter-. The sendmail version currently being developed is 8.6.5.
The mail agent most commonly used with is smail-3.1.28, written and copyrighted by Curt Landon Noll and Ronald S.-Karr. This is the one included in most distributions. In the following, we will refer to it simply as smail, although there are other versions of it which are entirely different, and which we don't describe here.
Compared to sendmail, smail is rather young. When handling mail for a small site without complicated routing requirements, their capabilities are pretty close. For large sites, however, sendmail always wins, because its configuration scheme is much more flexible.
Both smail and sendmail support a set of configuration files that have to be customized. Apart from the information that is required to make the mail subsystem run (such as the local hostname), there are many more parameters that may be tuned. sendmail's main configuration file is very hard to understand at first. It looks as if your cat had taken a nap on your keyboard with the shift key pressed. smail configuration files are more structured and easier to understand than sendmail's, but don't give the user as much power in tuning the mailer's behavior. However, for small UUCP or Internet sites the work required in setting up any of them is roughly the same.
In this chapter, we will deal with what email is and what issues you as an administrator will have to deal with. Chapters- and- will give instructions on setting up smail and sendmail for the first time. The information provided there should suffice to get smaller sites operational, but there are many more options, and you can spend many happy hours in front of your computer configuring the fanciest features.
Toward the end of the current chapter we will briefly cover setting up elm, a very common mail user agent on many ish systems, including .
For more information about issues specific to electronic mail on , please refer to the Electronic Mail HOWTO by Vince Skahan, which is posted to comp.os.linux.announce regularly. The source distributions of elm, smail and sendmail also contain very extensive documentation that should answer most of your questions on setting them up. If you are looking for information on email in general, there's a number of RFCs that deal with this topic. They are listed in the bibliography at the end of the book.