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IMAP on Linux: A Practical Guide

ABSTRACT: The Internet Mail Access Protocol, Version 4rev1 (IMAP4rev1), allows users to access and maintain hierarchical collections of e-mail folders on a remote server over the Internet. The "client-server" nature of the IMAP paradigm allows e-mail programs to enjoy the same benefits of portability and network transparency that graphical programs have gained from the X11 Windowing system. In this article, we describe how to set up client and server software on Linux to use IMAP for managing your mail. In addition, we explain the benefits and drawbacks of IMAP, and discuss when and under what situations it makes sense to use IMAP.

1. Why IMAP?

How do you read your e-mail today? Most likely, you start up a program like pine or Netscape to read your mail. You probably have only one Inbox for each e-mail account you own. Since a few month's worth of accumulated e-mail is much too unwieldy for a single Inbox, your mail messages are almost certainly organized into separate mail folders for easy cataloging and maintenance. Unless you use IMAP already, these mail folders are sitting on your local disk (or in your home directory on a remote account). However, there are a number of problems with storing mail folders on a local disk:

With a text-based mail client, you have to log in to the account that holds your mail folders in order to check your mail. This is not so bad if you have one account, but it can be tough juggling multiple accounts this way. For example, try moving a large number of messages from a folder in one account to a folder in another account.
With a graphical mail client, it can be difficult or impossible to manage your mail over a low bandwidth link. This point merits consideration since many people prefer graphical clients and have low bandwidth links.
It's not very easy to switch clients when all your mail folders are formatted for one particular program. Many users are finding it increasingly useful to be able to tailor their choice of mail client to best suit their current situation.

IMAP solves all these problems at once. The simple idea behind IMAP is that mail folders are stored on a central server and accessed via a commoditized, widely supported protocol. Using IMAP, you can:

access your mail folders from any machine, anywhere, as long as an IMAP client is installed,
manage multiple mail folders belonging to multiple e-mail accounts from a single client,
switch mail clients (Netscape, pine, Eudora) at will, and automatically carry all your mail folders with you.

The analogy to the X11 windowing protocol is helpful. In MS Windows, a graphical program running on a computer is inextricably bound to that computer's display. In contrast, under X, a program running on one machine can display itself on another machine through a well defined, commoditized protocol. The resulting network transparency is a critical advantage in today's highly interconnected world. IMAP offers the same kind of flexibility: your e-mail folders (that is, all the data you really care about) are stored on a central server, so that instead of being inextricably bound to one mail program on one machine, they can be transparently accessed over the network by any compliant program.

2. IMAP Server Installation

So now you're psyched about IMAP and want to use it, right?

The first step is to install an IMAP server. If your ISP already runs an IMAP server for you, then you might want to just use their server instead. An advantage of this route is that you can access your mail from anywhere without requiring your computer to be on. A disadvantage is that you have to dial in to your ISP to access your mail. In any case, most ISPs don't provide IMAP services, so you'll most likely have to run IMAP on your own computer anyway.

Without further ado, here's a quick and dirty set of instructions for installing the University of Washington IMAP server.

First, get and extract the latest version (4.4 as of this writing):

[root@localhost ~]# lynx ftp://ftp.cac.washington.edu/imap/imap-4.4.tar.Z
[root@localhost ~]# tar xzvf imap-4.4.tar.Z
[root@localhost ~]# cd imap-4.4

Type one of "make lnx", "make sl5", "make slx". The first is for traditional systems, the second is for systems using libc5 and shadow passwords, and the third is for glibc-based systems that use shadow passwords.

[root@localhost imap-4.4]# make lnx

Install the newly compiled file:

[root@localhost imap-4.4]# install -s -m 755 -o root -g mail imapd/imapd /usr/sbin

Add the following line to your /etc/inetd.conf (it may already be there; if so, uncomment it out):

imap    stream  tcp     nowait  root    /usr/sbin/tcpd  /usr/sbin/imapd

Set up your hosts.allow and hosts.deny files to restrict IMAP access to authorized domains only. This step is highly recommended, as the University of Washington IMAP server has had some fairly serious security vulnerabilities in the past.

In /etc/hosts.deny add the line

imapd: ALL

In /etc/hosts.allow add the machines and domains that you want to allow to access your IMAP server:

imapd: your.local.host.com
imapd: .yourisp.com
imapd: .yourschool.edu

Finally, restart inetd and your server is ready to go:

[root@localhost ~]# killall -HUP inetd

2.1. Distribution-Specific Installation Instructions

If you are running a Linux distribution that comes with a package manager, you can install a precompiled IMAP server if you want.

RedHat 5.2 instructions:

lynx ftp://ftp.redhat.com/pub/redhat/redhat-5.2/i386/RedHat/RPMS/imap-4.4-2.i386.rpm
rpm -Uvh imap-4.4-2.i386.rpm

Debian 2.0 instructions:

lynx ftp://ftp.debian.org/debian/dists/stable/main/binary-i386/mail/imap_4.2-1.deb
dpkg -i imap_4.2-1.deb

After installing these packages, you'll still have to go back and edit /etc/inetd.conf, /etc/hosts.deny, and /etc/hosts.allow yourself as described above.

3. IMAP Client Configuration

Once you've set up your server, configuring an IMAP client to use the server is a snap. The basic procedure is:

Pick a directory on the server system to hold all your mail folders. You need to have read and write access to this directory. I usually use $HOME/Mail. Create this directory if it doesn't exist.
Tell your IMAP client the name of your IMAP server, your username on that server, and the directory above where your mail folders live.
Now you can create and delete remote folders, and move messages to and from remote folders, just as if they were local folders, using the same techniques that you already use in your mail program to manipulate local folders.

Here's three examples of programs that I actually use:

3.1. Pine 4.05

Pine is available from http://www.washington.edu/pine/. It is very popular in the Unix world. The 4.0x versions added support for online IMAP folder access. To configure pine, press S to enter Setup, L to configure your collection list, and then A to add a collection. Enter your server, username, and mail folder directory as described above.

Simple, isn't it? Pine supports multiple IMAP collections, so you can add as many as you want and manage them all from one place.

Screenshot of pine configuration

3.2. Netscape Communicator 4.07

Netscape Communicator is an integrated web browser and Mail/News reader that is in fairly widespread use today. The 4.07 version is suitable for light mail processing, but it will crash if you give it a folder with well over 1000 messages (try it). Netscape Communicator is available from http://home.netscape.com/.

To set Netscape up for IMAP, select Preferences under the "Edit" menu, expand the "Mail & News" tab, click on the "Mail Server" entry, and enter in your username and your IMAP server. Obviously, make sure the server type "IMAP4" is selected. Click on the "More Options" box and enter in the mail folder directory you selected above. Finally, make sure the "Move Deleted Messages to Trash" box is not checked; this feature is rather broken and IMAP already provides flags to deal with deleted messages.

Netscape 4.0x does not support multiple IMAP collections, and it cannot automatically copy sent mail to a remote IMAP folder. Netscape 4.5 does support these things, but I have found the IMAP client in Netscape 4.5 to be far too unstable for real work.

Screenshot of Netscape configuration

3.3. TkRat 1.2

TkRat is my favorite graphical mail client right now. It also happens to be the only Open Source IMAP client I know (it's licensed under a BSD style license). It is available from http://www.dtek.chalmers.se/~maf/ratatosk/.

In TkRat, select "New/Edit Folder" from the Admin menu. Then select "IMAP Folders" from the Import menu, and type in your username, IMAP server, and a wildcard matching the folders in your mail folder directory. Note that TkRat expects a wildcard rather than a directory.

Screenshot of TkRat configuration

4. Important Usage Notes

Here's some things about IMAP that are not obvious, but are very useful to know.

4.1. Folder hierarchies

Currently, a limitation of the UW IMAP server is that a folder cannot contain both messages and subfolders. That is, a folder can either contain subfolders, or messages, but not both. To specify a folder that contains subfolders, you need to add a / to the end of its name.

Here's some examples:

Courses/ is a folder that can only contain subfolders.
Courses/Calculus is a subfolder of Courses/. It can only contain messages.
Courses/Languages/ is a subfolder of Courses/ that can only contain further subfolders.

4.2. The Inbox

The folder name INBOX, Inbox, or any capitalization thereof, is reserved for your inbox. You can't create a folder of your own with this name.

5. Security Considerations

Running an IMAP server adds another system daemon, and thus, another potential security vulnerability. If you're not going to make use of the capabilities of IMAP, you're probably better off not installing it.

A separate issue is the use of plaintext passwords for logins and authentication. Like most services, IMAP sessions are sent as plaintext over the Internet. Many people feel that sending passwords over the Internet as plaintext is no big deal. These people tend to use telnet, ftp, POP3, etc. without reservations. However, if you don't like sending your password over the Internet unprotected, you have precious few options:

Use Netscape Messaging Server, which supports IMAP over SSL. Unfortunately, there's no Linux version available, and the software costs $1295 besides.
Compile the Cyrus IMAP Server with Kerberos authentication support.
Use Secure Shell to transmit your IMAP session over an encrypted tunnel.
Install imp (a web-to-IMAP gateway) on your machine and access it through an SSL web server.

Unfortunately, all of these techniques are beyond the scope of this article. The fact of the matter is, most of the data on the Internet is transmitted as plaintext these days. If it were easy to conceal this data, people would be doing it already.

6. Conclusion

Fewer and fewer people are able to handle their daily volume of e-mail from one client on one machine all the time. While many are dealing with the e-mail mobility problem using the existing infrastructure of telnet, remote X displays, and distributed file systems, IMAP alone offers a comprehensive, application level solution tailored specifically for this need. By offering network transparency without sacrificing functionality, IMAP promises to revolutionize mobile mail access and change the way we read our mail for the better. I except that user demand will soon force IMAP support to be a required feature on all mail clients.

In short, if you're really happy with the way you read your mail now, then you don't need to bother with IMAP, but if you're itching for some additional flexibility in managing your mail, you should definitely consider adopting IMAP.