The Courier IMAP server

Courier-IMAP

For a general introduction and configuration settings for some popular IMAP clients, go and read imap/README(.html).

In this document:

Requirements

Now is the good time to read the FAQ, before you start. The FAQ is located in the file imap/FAQ(.html?).

UPGRADING

Upgrading from Courier-IMAP 4.14, and earlier

Version 4.15 removes the TLS_DHCERTFILE parameter from imap, and pop3d configuration files. DH parameters, and DH parameters only, get read from the new TLS_DHPARAMS file (and the other functionaly of TLS_DHCERTFILE, for DSA certificates, is merged into TLS_CERTFILE). After upgrading, run the mkdhparams script to create a new TLS_DHPARAMS file.

Upgrading from Courier-IMAP 4.9.3, and earlier

In 4.10.0, the IMAP server resets the epoch for an internal sequence number generator for new mailboxes. This is an internal attribute of individual IMAP folders, that's defined by the IMAP specification. Each folder in a mailbox carries an individual sequence number, it is defined as a 32 bit integer value, and required to be a monotonically increasing value. and RFC 2060 recommended that "... a good value to use for the unique identifier validity value is a 32-bit representation of the creation date/time of the mailbox."

On modern platforms, the system time is now a 64 bit value (even on the remaining 32 bit platforms). With Y2038K on the horizon, it's time to reset the epoch (the new epoch, for anyone who cares, runs until the year 2069). The upgrade impact on existing systems is as follows.

There is no impact on existing folders in existing mailboxes. New folders will have their internal sequence number in the new epoch.

One potential issue exists if a folder gets deleted by the IMAP client, and then recreated later. The new folder will now get a lower sequence number. Although this is technically not allowed, it's unlikely to cause problems with most IMAP clients. If the same IMAP client deletes and recreated the mailbox, the client should be completely up to speed. If, however, there's an IMAP client that accesses the same folder, and some other IMAP client deletes and recreates the same folder, this might cause confusion. Most IMAP clients are likely to recover automatically; most IMAP clients only care that the new sequence number they see is different from the previous one, in order to trigger a full resynchronization with the server. In case an IMAP client fails to resynchronize, the remedy is to remove the IMAP account configuration from the client, and add it back in.

Copying a mailbox by directly copying the files in maildirs preserves each folder's epoch. However if a mailbox gets migrated by copying its contents over IMAP, the folders on the destination IMAP server will not necessarily have a monotonically higher value -- neither does IMAP guarantee that different IMAP servers must be in agreement with each other on the subject of sequence numbers -- and if IMAP clients are repointed to a new server they may experience problems opening existing mailboxes. To remedy this situation it will be necessary to completely remove and then reconfigure the IMAP account, in the IMAP client. Again, verbatim copying of maildirs has no issues.

A marginal situation exists where if a server completely runs of disk space, or if there's a hardware failure, and the IMAP server is unable to retrieve or save an existing folder's sequence number, and must now start afresh and generate a new one, the IMAP server running on a new epoch will recover with a lower sequence than the one that existed before. The rememdy is the same: remove the IMAP account configuration from the client, and then recreate it.

Upgrading from Courier-IMAP 3, and earlier.

Beginning with 4.0, the authentication library that used to be a part of Courier-IMAP's source has been spun off into a standalone authentication library.

You must download and install the Courier Authentication Library from http://www.courier-mta.org/authlib/ before upgrading. Review the documentation in the courier-authlib package for more information.

After upgrading to 4.0, or later, to avoid future confusion the old copies of these configuration files (including the .dist files), should be removed from Courier-IMAP's configuration directory. They now live in Courier-authlib's configuration directory (/usr/local/etc/authlib, or whatever was specified to Courier-authlib's configure script).

Upgrading from Courier-IMAP 1.7.3, and earlier.

After upgrading from Courier-IMAP 1.7.3, or earlier, any existing mail in POP3 mailboxes may show up as new mail, by some mail clients. Other mail clients may end up downloading a second copy of any message that was left in the mailbox before the upgrade. This is a one-time event. Courier-IMAP 2.0.0 uses a different mechanism for generating POP3 message identifiers. Mail clients that use POP3 identifiers will behave as if all messages, that were left in the POP3 mailbox before the upgrade, were removed, and replaced by new messages that happen to be the same content. Depending on how the POP3 mail client works, it will either flag all messages in the mailbox as unread, or download a second copy of the message.

Upgrading from Courier-IMAP 1.3.0, and later versions, is a straightforward process. Follow the instructions in the INSTALLATION section, below, to install the new version. The "make install-configure" command automatically preserves the existing system configuration. However, note that new versions of Courier-IMAP will often introduce additional configuration options. After make install-configure a cursory inspection of configuration files in /usr/lib/courier-imap/etc (the default location of the configuration directory) is recommended, in order to identify any new configuration settings that might need adjustment.

Upgrading from Courier-IMAP 1.3.8.2 and earlier

The default configuration options have slightly changed. The default configuration script will now always build the authdaemon module, and build all real authentication modules inside authdaemond. This is true even with the authvchkpw module.

Upgrading from Courier-IMAP 1.2.3 and earlier

Courier-IMAP 1.3.0 introduced a new configuration file format that allows configuration files to be automatically upgraded. Additionally, several existing configuration files have been renamed in order for their names to be consistent with the Courier build:

Courier-IMAP < 1.3  Courier-IMAP 1.3.0
--------            ---------
imapd.config        imapd
imapd-ssl.config    imapd-ssl
pop3d.config        pop3d
pop3d-ssl.config    pop3d-ssl

The NEWS file has a detailed explanation of how configuration files are now installed. Basically, make install now installs configfilename.dist, and make install-configure copies configfilename.dist to configfilename, becoming the actual configuration file. If there is an existing configfilename, the old settings in configfilename which are still valid will be kept in the new configfilename.

This only works as long as both the old and the new configuration files are in the new format, so this will actually take effect with your next upgrade Courier-IMAP. If the previous installed version of Courier-IMAP did not use the new format for configuration files (1.2.3 and earlier), the old configuration file is backed up to configfilename.bak.

The recommended procedure for upgrading from versions 1.2.3 and earlier is as follows:

The recommended upgrade procedure is as follows:

All configuration files are kept in the configuration directory. Nothing else in /usr/lib/courier-imap is configurable. Do not simply overwrite 1.3.0 configuration files with configuration files from the previous version. It's tempting, but don't do it. It may work, but you will lose the automatic upgrade capability for future releases.

Upgrading from Courier-IMAP 1.1 or earlier

Note that Courier-IMAP 1.2 includes a compatible POP3 server, and the installation script will also install a POP3 server on your system. Even though it is installed, you are not required to use it, but you still need to be aware of its existence. If you install the RPM build of Courier-IMAP, you're going to get the POP3 server started at system boot. If you do not need POP3 services, edit both the pop3d.config and pop3d-ssl.config configuration files, and set POP3DSTART and POP3DSSLSTART to NO

Upgrading from Courier-IMAP 1.0 or earlier

If the server is running, manually stop the server before installing the new version.

INSTALLATION

To compile and install the Courier-IMAP server (this is the short version, a longer version follows):

 
$ ./configure [ options, see below ]
$ make
$ make check       # Note - the --enable-workarounds-for-imap-client-bugs
                   # option to configure will result in make check FAILING.
$ su root
# make install     # Or, make install-strip, to strip the executables.
# make install-configure   # Install configuration files.

                   # Start the authdaemond process

NOTE

You MUST run the configure script as normal user, not root. Did you extract the tarball as root? It won't work. Remove the extracted source code. Log in as a normal user. Extract the source code as a normal user, then run configure. You will do everything as a normal user, except for the final step of installing the compiled software.

NOTE

Courier-IMAP does not use inetd or xinetd. Any inetd or xinetd configuration settings for the IMAP and POP3 ports must be turned off. Courier-IMAP will not start if inetd or xinetd is listening for IMAP or POP3 connections.


As mentioned in "Requirements", above, if you are using xBSD, you must use gmake instead of make.


NOTE: The configure script may run as much as 5-10 minutes on slow machines. It may appear that configure is stuck in a loop, but that's an illusion. Courier-IMAP is built from a collection of modular components, each with its own configuration script. The configuration scripts share a lot of common code, leading to an initial impression that the same configuration script is being repeatedly run.

See below for a description of the options to the configure script.

WARNING: set your umask to 022 before running make install or make install-strip.

You should try make install-strip first. Use make install if make install-strip fails.

The configure script accepts certain options, but the defaults should be fine most of the time. make install puts everything in /usr/lib/courier-imap. If the directory /etc/pam.d exists, make install creates /etc/pam.d/imap and /etc/pam.d/pop3, overwriting any existing files. If you have some other IMAP server installed, this means that you will want to save your existing configuration in /etc/pam.d/{imap|pop3}.

"make check" performs some internal sanity checks. If make check fails, something is wrong, and Courier-IMAP may not work for you reliably. Certain options are documented to cause make check to fail, due to different IMAP protocol behavior. If you need to use those options, first compile Courier-IMAP without them, run make check, and if all goes well extract the source code again in a different directory, then build it for the second time using your options.

After installation, you will need to review the files in /usr/lib/courier-imap/etc and make any changes you deem necessary.

After running make install or make install-strip you will then have to modify your system's startup scripts to run Courier-IMAP when your system boots.

Use the following command to start the Courier-IMAP server:

 
$ /usr/lib/courier-imap/libexec/imapd.rc start

This assumes that Courier-IMAP is installed in /usr/lib/courier-imap. Use the following command to stop Courier-IMAP:

 
$ /usr/lib/courier-imap/libexec/imapd.rc stop

You will have to add these commands to your system startup/shutdown scripts.

IMAP over SSL

To add SSL support you have to install OpenSSL or GnuTLS before installing Courier-IMAP. Download OpenSSL from http://www.openssl.org/, or GnuTLS from http://www.gnutls.org.

Follow OpenSSL's or GnuTLS's installation instructions, then build Courier-IMAP.

NOTE: Most systems already have an available OpenSSL or GnuTLS package. Do not build OpenSSL or GnuTLS yourself, if a prebuilt package is already available. Just install the prebuilt package.

NOTE: The development libraries must be installed in addition to the runtime package, in order to build Courier-IMAP. On most systems, the development files (header files, libraries, etc...) are provided in a separate "devel" package. The base OpenSSL/GnuTLS package is not sufficient to build Courier-IMAP, the development libraries must be installed.

The OpenSSL library is selected when both OpenSSL and GnuTLS libraries are found by the configure script. Use the --with-gnutls option to explicitly select the GnuTLS library over OpenSSL.

The /usr/lib/courier-imap/etc/imapd-ssl configuration file sets some additional options for SSL support, which you may need to adjust. Consult that configuration file for additional information. Then, you also have to run the /usr/lib/courier-imap/libexec/imapd-ssl.rc script from your system startup and shutdown scripts, just like the /usr/lib/courier-imap/libexec/imapd.rc script. You may accept both SSL and non-SSL connections by running both scripts.

Note that SSL requires a valid, signed, X.509 certificate to be installed where Courier-IMAP expects to find it. The default location for the X.509 certificate, in PEM format, is /usr/lib/courier-imap/share/imapd.pem. The X.509 certificate must be signed by a certificate authority that is known to the IMAP client. You can generate your own self-signed certificate by running the script /usr/lib/courier-imap/share/mkimapdcert which will work too, except that IMAP clients using SSL will display a warning message the first time they connect to the server. To get rid of the warning message you'll have to pay for a signed X.509 certificate. The gory details of setting up SSL is beyond the scope of this document, and you should consult the OpenSSL documentation for more information.

The mkimapdcert script will not overwrite an existing imapd.pem certificate, in order to allow precompiled packages to simply call mkimapdcert after installation, without worry.

Initial parameters, and a monthly cron job to generate DH parameters

Run the mkdhparams script to create a DH parameter file. A monthly cron job should be created to run the mkdhparams script, in order to periodically generate a new set of DH parameters. mkdhparams checks if the DH parameter file's timestamp is older than 25 days, and creates a new file if it is. DH parameters are used to set up encrypted connections.

The bundled POP3 server

The POP3 server included with Courier-IMAP provides POP3 access to INBOX, and that's about it. Enabling the POP3 server is very similar to enabling the IMAP server, with the following differences:

The configuration files are /usr/lib/courier-imap/etc/pop3dand /usr/lib/courier-imap/etc/pop3d-ssl.

The startup/shutdown scripts are /usr/lib/courier-imap/libexec/pop3d.rcand /usr/lib/courier-imap/libexec/pop3d-ssl.rc.

The SSL certificate is /usr/lib/courier-imap/share/pop3d.pem, and the /usr/lib/courier-imap/share/mkpop3dcert script can be used to create a self-signed SSL certificate for testing purposes.

System-V style startup

If your system uses System-V style startup scripts, take a look at courier-imap.sysvinit - this is a sample /etc/init.d script. courier-imap.sysvinit is created by configure. In most cases it can be merely copied to /etc/init.d and /etc/rc?.d directories (with the execute permission bit turned on).

The sample startup script will check if IMAP or POP3 over SSL is enabled. The sample startup script automatically creates dummy SSL certificates the first time it is executed.

Options to configure:

Installation directories

Unless the options --prefix, --bindir, or --mandir are used, everything will be installed in the directory /usr/lib/courier-imap.

Use the --prefix option to specify a different directory. This directory will have the following subdirectories:

Having everything installed underneath one directory allows its contents to be easily backed up, before a newer version of courier-imap is installed. Reverting to a previous version is as simple as restoring from backup.

Because some binaries in bin and sbin may be executed from the command line, it will be necessary to change your systemwide global startup script to add this directory to the default PATH. Additionally, it will also be necessary to modify the configuration of the man(1) command so that it can find Courier-IMAP's manual pages in this directory:

        PATH="/usr/lib/courier-imap/bin:$PATH"
        if test -w /etc
        then
                PATH="/usr/lib/courier-imap/sbin:$PATH"
        fi
        export PATH
        MANPATH="/usr/lib/courier-imap/man:$MANPATH"
        export MANPATH

As an alternative, you may use the --bindir and --mandir options in order to install binaries to /usr/local/bin and the manual pages to /usr/local/man, which should already be searched by default:

        ./configure --bindir=/usr/local/bin --mandir=/usr/local/man

Other familiar configure options, such as --sysconfdir and --datadir work too, for those who know how to properly use them.

ACCOUNT INITIALIZATION HOOK

If there is a file or a symbolic link in the maildir called "loginexec", and if it is executable, then the executable file will be invoked after a succesful login. If the program terminates with an exit code of 0, the "loginexec" file (or a symbolic link) will be removed.

USING SHARED FOLDERS

Courier-IMAP supports shared folders. See the file README.sharedfolders.html for information on how to set up shared folders.

CRAM-MD5 AUTHENTICATION

CRAM-MD5 authentication allows IMAP clients to authenticate themselves without sending the password in clear-text over the network. Courier-IMAP now supports CRAM-MD5 by default, but is not enabled for reasons explained below. CRAM-MD5 support is implemented by the authcram module, with one exception - authldap, authpgsql, and authmysql support CRAM-MD5 authentication if the LDAP or the MySQL/PostgreSQL server stores clear-text passwords, and not crypt-ed passwords.

To use CRAM-MD5 it is necessary to use an IMAP client that support CRAM-MD5 authentication, of course. That's the easy part.

The problem is that it is not possible to use the system password when logging in using CRAM-MD5. That's because CRAM-MD5 requires the knowledge of the actual password, in the clear, in order to calculate authentication tokens (even though that the password itself is not sent in the clear over the network).

So, implementation of CRAM-MD5 is an advanced task that should be attempted only when you are comfortable with, and fully understand how Courier-IMAP works in general. Here's an overview of this procedure:

Enabling CRAM-MD5 authentication

Because of these unfortunate complexities, CRAM-MD5 authentication is disabled after installation. When you're ready to use CRAM-MD5, edit the imapd configuration file and add the "AUTH=CRAM-MD5" keyword to the IMAP_CAPABILITY environment variable, then restart Courier-IMAP. There are instructions in the imapd configuration file to that effect.

If you do not intend to ever use CRAM-MD5 authentication, you can either specify --without-authcram option to the configure script, or simply edit imapd and remove authcram from the AUTHMODULES setting.

CERTIFICATE AUTHENTICATION

Courier-IMAP can use SSL certificates for authentication purposes. For certificate authentication purposes, one of the fields in your certificates' subject must match the login ID in the authentication database. Consider the following certificate:

...
Subject: C=US,ST=New York,L=New York,O=Acme Widgets Inc,CN=John Smith,emailAddress=johnsmith@example.com

If the emailAddress field is configured as the login ID, the authentication database must provide login details for johnsmith@example.com. To enable certificate authentication, edit the imapd-ssl and pop3d-ssl configuration files, and make the following changes:

SENDING MAIL VIA AN IMAP CONNECTION

This server allows using the IMAP connection to send E-mail. Normally, the IMAP protocol provides only access to mail in an existing mail account, and mail clients must use SMTP in order to send mail. The Courier-IMAP server has an optional setting to enable mail to be send via an IMAP connection in a manner that should work with all existing IMAP mail clients. This can be useful when an account is logged in from a shared access pool which normally blocks most access to the SMTP port.

This is implemented by enabling a setting in the imapd configuration file that designates a folder as a special "Outbox" folder. The default setting is a folder called "Outbox" (IMAP path INBOX.Outbox), but the name can be changed to anything. This folder, for the most part, is no different than any other folder. If a folder by that name doesn't exist, it needs to be created, just like any other IMAP folder. It looks and acts like any other folder, except that each message added to the folder, via IMAP's APPEND or COPY command, will also be mailed out by the Courier-IMAP server to the addresses listed in the To:, Cc:, and Bcc: headers.

It should be possible to use this to send mail from any IMAP client by:

  1. Composing a draft message, telling the IMAP client to save the draft message in its drafts folder on the IMAP server.
  2. Opening the drafts folder, and moving or copying the message to the Outbox folder.
  3. The act of copying the message into the Outbox folder will send the mail. There won't be any explicit notification to the fact that the message was sent, so it's a good idea to include your own E-mail address on the Cc: list.

NOTE: it is tempting to configure the IMAP mail client to use Outbox as its default folder for saving drafts. Resist the temptation. If you forget, you'll save a partially completed draft, which will be then obediently mailed out.

NOTE: the message, in addition to being sent, will be saved in the folder in the normal fashion. After saving the message, reopen the Outbox folder and delete the sent message, or move it someplace else.

NOTE: when enabled, the Courier-IMAP server will advertize a private XCOURIEROUTBOX IMAP capability. It is theoretically possible to code an IMAP mail client that reads this capability and automatically configures itself accordingly -- when this IMAP capability is present -- to send E-mail in the normal way but using the IMAP connection. At this time, I'm not aware of any actual mail clients that know how to do this.

NOTE: many mail clients save some additional internal information in headers of draft messages. The internal information is normally removed before the mail client sends the message. Make sure that none of this extra information is something that should not be mailed out.

REALTIME FOLDER STATUS UPDATES

If Gamin (http://www.gnome.org/~veillard/gamin/) or FAM (http://oss.sgi.com/projects/fam/) is installed it will be possible to allow multiple clients to open the same folder, and have all clients to be simultaneously notified of any changes to the folder contents.

After installing the server see the imapd(8) manual page for more information.

Account OPTIONS

If the option 'disableimap' or 'disablepop3' is set to a non-zero value, then logins via IMAP or POP3 respectively will be disabled for that account. You can use the DEFAULTOPTIONS setting to disable a service globally and then re-enable it for individual accounts; for example, setting DEFAULTOPTIONS="disableimap=1" will disable IMAP access for all accounts except those which have option disableimap=0

See README_authlib.html in the courier-authlib package for information on how to set per-account options.

SMAP

Starting with Courier-IMAP 2.0, the server supports an experimental mail access protocol, dubbed "Simple Mail Access Protocol". SMAP is an experiment to provide enhanced mail processing beyond what's currently possible with IMAP. SMAP's purpose is to prototype and develop advanced mail access functionality that's not possible with IMAP. SMAP is disabled by default. Uncomment the SMAP_CAPABILITY setting in the imapd configuration file in order to enable SMAP. The Cone mail client supports SMAP.