A few comments on my article The perfect network server in issue 17 requested some more in depth follow-up pieces. This is what I hope to be the first of those. It focuses on Exim, the mail transfer agent (MTA), specifically setting it up with spam scanning. It is based on setups I currently use, hosted on Debian GNU/Linux.

This is an intermediate article, I’m going to assume you are familiar with mail delivery technologies and terminology. If you don’t know what SMTP, MX records and reverse DNS lookups are, you might like to do a little background reading before coming back to this article. I’ll try to stick to a descriptive narrative style but some of it will inevitably involve technical language.

What Exim is not

Exim is a mail transfer agent. It receives messages, usually by SMTP, figures out what to do with them according to its configuration and transfers them to another location based on that information. The new location may be a locally based mailbox, another server or another daemon running on the same box.

Exim does not do POP3, IMAP, shared calendars or make the tea

It does not deliver mail to client machines, and does not handle mail user agent message creation functionality. In short, Exim does SMTP and related stuff. It does not do POP3, IMAP[1], LDAP, shared calendars or make the tea. It’s important to note that Exim is not capable of pulling mail: it expects all messages to be delivered to it [2].

Debian Exim packages

Debian has a few packages for Exim. At the time of writing the current major version is 4. The key packages to install are exim4-base, exim4-config and one of the two exim4-daemon-? packages. If you are installing a brand new server, then you can install Exim as a task during the Debian installation stage (see The perfect network server for information on that). If you already have a Debian server then you can install it using tasksel install mail-server or with your favourite package manager.

The two Exim4 daemon packages within Debian called exim4-daemon-light and exim4-daemon-heavy. Installing either will install the exim4-base package as a dependency but not exim4-config. So, you should install exim4-config as well as it comes in handy.

The light daemon package is a perfectly adequate Exim install but leaves out things like SQL data lookups, virus/spam scanning integration and Secure Password Authentication SMTP. All of those are included in the “heavy” daemon and, as you’ll want to do spam scanning, you’ll need it. Note that tasksel, and thus the Debian installer, will install exim4-daemon-light but you can replace it simply by installing the heavy package afterwards. So, after all that, apt-get install exim4-daemon-heavy exim4-config does the trick.

Exim initial configuration

I’m not aware of any GUI tools to configure Exim, beyond address and account management. I started a fairly long debate on the necessity of server GUIs in the last article, but as they say, “write what you know”… so I’ll stick to configuring Exim via the shell.

Having installed exim4-config, you might as well put it to good use

Having installed exim4-config, you might as well put it to good use. To use exim-config, run (either as root or sudo) the command dpkg-reconfigure exim4-config. I won’t detail every step of this because each has decent explanatory text within—and I’d take up most of the article installing Exim. Here are a few pointers on the latter steps.

The debconf way to set-up Exim

Mailbox format

The Exim local_delivery transport is used to deliver messages to local mailboxes. By default that is in mbox format. There is an option to use the popular Maildir format, which you can set here. I prefer Maildir, so I choose to use it here. For a discussion on which is best I suggest you put your flame-proof suit on and hit Google or add a comment. Briefly, mbox stores all messages in a single text file. Maildir stores each message as an individual file within sub-directories of your main maildir. An advantage of Maildir is that it doesn’t require file locking, so there are fewer delays.