Sebastopol, CA--It's not a new story, especially in open source circles--that of a software program that gains widespread use and practically takes on a life of its own apart from its creator. In the case of Exim, the popularity and endurance of the program are particularly telling of its merits. Philip Hazel, author of Exim: The Mail Transfer Agent (O'Reilly, US $44.95), began building Exim from scratch in 1995 when it became clear to him that the system they were using at the University of Cambridge no longer met their requirements. As Hazel explains, "Up to that time, the Internet had been a pretty friendly place, and there was little need to take many precautions against hostile acts. Most sites ran open mail relays, for example. It was clear, however, that this situation was changing." Because he wasn't exactly sure what the outcome would be, he called his program the EXperimental Internet Mailer, or Exim.
Hazel shared his creation with one of his colleagues, who began telling others about it. "I put it on an ftp site just in case anybody else was interested, but never advertised it," Hazel says, "I am somewhat amazed at how widely it has spread, and how many different types of systems it is used on." The early releases of Exim were never "announced;" they just spread by word of mouth. Today Exim is the default mail transfer agent (MTA) installed on some Linux systems, runs on many versions of Unix, and is suitable for any TCP/IP network with any combination or hosts and end-user software. Exim continues to grow in popularity because it is open source, scalable, rich in features, and easy to configure.
A mail transfer agent, known interchangeably as a "mail transport agent" is responsible for routing and delivering electronic mail. Every local area network and any multi-user system requires an MTA to deliver local mail after it reaches the system. Exim, qmail, and sendmail are examples of MTAs. While sendmail is by far the most popular MTA on Unix and Linux systems, Exim is rapidly gaining ground because it is simpler and more intuitive to configure than sendmail, and offers all the features and flexibility needed by most Internet-connected sites.
Although Hazel has made a point of maintaining a comprehensive reference manual for Exim, the need for introductory and tutorial material has become apparent. As Hazel says, "Some Linux distributions are now shipping with Exim. This means that less-experienced system administrators are having to learn about it. The official specification is written as a reference manual, which is not the best format for learning, especially if you have no previous experience of mail transfer agents. Exim: The Mail Transfer Agent contains much more introductory material than the reference manual, and is presented in a better order for reading through."
Exim's features include sophisticated rules for routing mail to individual users or sets of users, information lookups in a variety of formats, spam filtering, virus or attachment checking, and mailing list management. Exim: The Mail Transfer Agent was written in particular for Unix system administrators, but should be valuable to anyone who maintains an Exim installation or is considering running Exim as a replacement for an existing MTA.
Chapter 3, "Exim Overview," is available free online.
More information about the book, including Table of Contents, index, author bio, and samples.
A cover graphic in jpeg format.
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