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The Exim SMTP mail server

Philip Hazel
Published by UIT Cambridge
616 pages
£ 37.50
Published: August 2007
reviewed by Alain Williams
   in the December 2007 issue (pdf), (html)

Phil Hazel is the author of Exim and writes in a dry but precise style.

Exim is one of the big 4 Unix MTAs (Mail Transfer Agents) in use today, this book will help you get going and then guide you through some of the more interesting (read: complicated) set ups that you might have.

It starts with an overview of how email hangs together (MUAs, MTAs, MX records) and enough of the SMTP protocol to understand what you are doing as a postmaster. Chapters 3 to 5 give the big picture of how Exim works with configuration fragments to illustrate points. Typical needs are presented as examples such as modifying message bodies, scanning for virus, aliases, mail lists, virtual domains. If you are new to Exim I would advise you to read these first 99 pages as a novel -- straight through; the rest of the book expands on this and you could pick out bits as you need them.

The next 100 odd pages deal with routers (that decide where to send mail) and transports (that specify how to get it there). It starts with generic options followed by those specific to a particular router or transport. The style is to explain why an option is needed followed by examples and often a discussion on (dis)advantages of a particular approach in different circumstances. These chapters are important as they explain why Exim users love its flexibility (I admit to being an Exim fan).

Next comes mail filtering, either by something like procmail or by Exim's built in mechanism. Then how to read the log files, how to control retrying in the event of a delivery failure, authentication and SMTP encryption.

Some 60 pages of email protection follow: how to verify the remote host and sender, spam and virus scanning, stopping open relays. This is all tied together by the all important Access Control Lists. ACLs are are flexible but take a bit of getting your head around if you want to write your own. They are well explained, but this is a tough chapter, partly because ACLs touch many different aspects of Exim's operation.

After header processing and address rewriting comes lookups. This important feature allows Exim to transform an email address by a lookup in a file, database, NIS, LDAP, ... You will need to supplement what you learn here with perusal of details in the reference manual.

The last 150 pages describes config file syntax and string manipulation; then the command line and hints on how to administer Exim.

The definitive guide to Exim remains the reference manual, some rarely used options are not in this book.

This is the 2nd edition of this book, my first edition is well thumbed. This is not a rewrite, but reflects improvements to Exim, in particular the new content scanning mechanism and updates on access control, address verification and address and header processing. The first edition was published in 2003 and was 595 pages long.

Summary: if you use Exim in a serious way then you need this book.

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