Hooking up your mail server

Mailman needs to communicate with your MTA (mail transport agent or mail server, the software which handles sending mail across the Internet), both to accept incoming mail and to deliver outgoing mail. Mailman itself never delivers messages to the end user. It sends them to its immediate upstream MTA, which delivers them. In the same way, Mailman never receives mail directly. Mail from outside always comes via the MTA.

Mailman accepts incoming messages from the MTA using the Local Mail Transfer Protocol (LMTP) interface. LMTP is much more efficient than spawning a process just to do the delivery. Most open source MTAs support LMTP for local delivery. If yours doesn’t, and you need to use a different interface, please ask on the mailing list or on IRC.

Mailman passes all outgoing messages to the MTA using the Simple Mail Transfer Protocol (SMTP).

Cooperation between Mailman and the MTA requires some configuration of both. MTA configuration differs for each of the available MTAs, and there is a section for each one. Instructions for Postfix and Exim (v4) are given below. We would really appreciate a contribution of a configuration for Sendmail, and welcome information about other popular open source mail servers.

Configuring Mailman to communicate with the MTA is straightforward, and basically the same for all MTAs. Here are the default settings; if you need to change them, edit your mailman.cfg file:

[mta]
incoming: mailman.mta.postfix.LMTP
outgoing: mailman.mta.deliver.deliver
lmtp_host: 127.0.0.1
lmtp_port: 8024
smtp_host: localhost
smtp_port: 25
configuration: python:mailman.config.postfix

This configuration is for a system where Mailman and the MTA are on the same host.

Note that the modules that configure the communication protocol (especially incoming) are full-fledged Python modules, and may use these configuration parameters to automatically configure the MTA to recognize the list addresses and other attributes of the communication channel. This is why some constraints on the format of attributes arise (e.g., lmtp_host), even though Mailman itself has no problem with them.

It is possible (although not documented here) to completely replace or override the default mechanisms to handle both incoming and outgoing mail. Mailman is highly customizable here!

The incoming and outgoing parameters identify the Python objects used to communicate with the MTA. The python: scheme indicates that the paths should be a dotted Python module specification. The deliver module used in outgoing should be satisfactory for most MTAs. The postfix module in incoming is specific to the Postfix MTA. See the section for your MTA below for details on these parameters.

lmtp_host and lmtp_port are parameters which are used by Mailman, but also will be passed to the MTA to identify the Mailman host. The “same host” case is special; some MTAs (including Postfix) do not recognize “localhost”, and need the numerical IP address. If they are on different hosts, lmtp_host should be set to the domain name or IP address of the Mailman host. lmtp_port is fairly arbitrary (there is no standard port for LMTP). Use any port convenient for your site. “8024” is as good as any, unless another service is using it.

smtp_host and smtp_port are parameters used to identify the MTA to Mailman. If the MTA and Mailman are on separate hosts, smtp_host should be set to the domain name or IP address of the MTA host. smtp_port will almost always be 25, which is the standard port for SMTP. (Some special site configurations set it to a different port. If you need this, you probably already know that, know why, and what to do, too!)

Mailman also provides many other configuration variables that you can use to tweak performance for your operating environment. See the src/mailman/config/schema.cfg file for details.

Postfix

Postfix is an open source mail server by Wietse Venema.

Mailman settings

You need to tell Mailman that you are using the Postfix mail server. In your mailman.cfg file, add the following section:

[mta]
incoming: mailman.mta.postfix.LMTP
outgoing: mailman.mta.deliver.deliver
lmtp_host: mail.example.com
lmtp_port: 8024
smtp_host: mail.example.com
smtp_port: 25

Some of these settings are already the default, so take a look at Mailman’s src/mailman/config/schema.cfg file for details. You’ll need to change the lmtp_host and smtp_host to the appropriate host names of course. Generally, Postfix will listen for incoming SMTP connections on port 25. Postfix will deliver via LMTP over port 24 by default, however if you are not running Mailman as root, you’ll need to change this to a higher port number, as shown above.

Basic Postfix connections

There are several ways to hook Postfix up to Mailman, so here are the simplest instructions. The following settings should be added to Postfix’s main.cf file.

Mailman supports a technique called Variable Envelope Return Path (VERP) to disambiguate and accurately record bounces. By default Mailman’s VERP delimiter is the + sign, so adding this setting allows Postfix to properly handle Mailman’s VERP’d messages:

# Support the default VERP delimiter.
recipient_delimiter = +

In older versions of Postfix, unknown local recipients generated a temporary failure. It’s much better (and the default in newer Postfix releases) to treat them as permanent failures. You can add this to your main.cf file if needed (use the postconf command to check the defaults):

unknown_local_recipient_reject_code = 550

While generally not necessary if you set recipient_delimiter as described above, it’s better for Postfix to not treat owner- and -request addresses specially:

owner_request_special = no

Transport maps

By default, Mailman works well with Postfix transport maps as a way to deliver incoming messages to Mailman’s LMTP server. Mailman will automatically write the correct transport map when its mailman aliases command is run, or whenever a mailing list is created or removed via other commands. Mailman supports two type of transport map tables for Postfix, namely hash and regexp. Tables using hash are processed by postmap command. To use this format, you should have postmap command available on the host running Mailman. It is also the default one of the two. To connect Postfix to Mailman’s LMTP server, add the following to Postfix’s main.cf file:

transport_maps =
    hash:/path-to-mailman/var/data/postfix_lmtp
local_recipient_maps =
    hash:/path-to-mailman/var/data/postfix_lmtp
relay_domains =
    hash:/path-to-mailman/var/data/postfix_domains

where path-to-mailman is replaced with the actual path that you’re running Mailman from. Setting local_recipient_maps as well as transport_maps allows Postfix to properly reject all messages destined for non-existent local users. Setting relay_domains means Postfix will start to accept mail for newly added domains even if they are not part of mydestination.

Note that if you are not using virtual domains, then relay_domains isn’t strictly needed (but it is harmless). All you need to do in this scenario is to make sure that Postfix accepts mail for your one domain, normally by including it in mydestination.

Regular Expression tables remove the additional dependency of having postmap command available to Mailman. If you want to use regexp or Regular Expression tables, then add the following to Postfix’s main.cf file:

transport_maps =
    regexp:/path-to-mailman/var/data/postfix_lmtp
local_recipient_maps =
    regexp:/path-to-mailman/var/data/postfix_lmtp
relay_domains =
    regexp:/path-to-mailman/var/data/postfix_domains

You will also have to instruct Mailman to generate regexp tables instead of hash tables by adding the following configuration to mailman.cfg:

[mta]
incoming: mailman.mta.postfix.LMTP
outgoing: mailman.mta.deliver.deliver
lmtp_host: mail.example.com
lmtp_port: 8024
smtp_host: mail.example.com
smtp_port: 25
configuration: /path/to/postfix-mailman.cfg

Also you will have to create another configuration file called as postfix-mailman.cfg and add its path to the configuration parameter above. The postfix-mailman.cfg would look like this:

[postfix]
transport_file_type: regex

Postfix documentation

For more information regarding how to configure Postfix, please see the Postfix documentation at:

Exim

Exim 4 is an MTA maintained by the University of Cambridge and distributed by most open source OS distributions.

Mailman settings

Add or edit a stanza like this in mailman.cfg:

[mta]
# For all Exim4 installations.
incoming: mailman.mta.exim4.LMTP
outgoing: mailman.mta.deliver.deliver
# Typical single host with MTA and Mailman configuration.
# Adjust to your system's configuration.
# Exim happily works with the "localhost" alias rather than IP address.
lmtp_host: localhost
smtp_host: localhost
# Mailman should not be run as root.
# Use any convenient port > 1024.  8024 is a convention, but can be
# changed if there is a conflict with other software using that port.
lmtp_port: 8024
# smtp_port rarely needs to be set.
smtp_port: 25
# Exim4-specific configuration parameter defaults.  Currently empty.
configuration: python:mailman.config.exim4

For further information about these settings, see mailman/config/schema.cfg.

Exim4 configuration

The configuration presented below is mostly boilerplate that allows Exim to automatically discover your list addresses, and route both posts and administrative messages to the right Mailman services. For this reason, the mailman.mta.exim4 module ends up with all methods being no-ops.

This configuration is field-tested in a Debian “conf.d”-style Exim installation, with multiple configuration files that are assembled by a Debian-specific script. If your Exim v4 installation is structured differently, ignore the comments indicating location in the Debian installation.

# /etc/exim4/conf.d/main/25_mm3_macros
# The colon-separated list of domains served by Mailman.
domainlist mm_domains=list.example.net

MM3_LMTP_PORT=8024

# MM3_HOME must be set to mailman's var directory, wherever it is
# according to your installation.
MM3_HOME=/opt/mailman/var
MM3_UID=list
MM3_GID=list

################################################################
# The configuration below is boilerplate:
# you should not need to change it.

# The path to the list receipt (used as the required file when
# matching list addresses)
MM3_LISTCHK=MM3_HOME/lists/${local_part}.${domain}

# /etc/exim4/conf.d/router/455_mm3_router
mailman3_router:
  driver = accept
  domains = +mm_domains
  require_files = MM3_LISTCHK
  local_part_suffix_optional
  local_part_suffix = \
     -bounces   : -bounces+* : \
     -confirm   : -confirm+* : \
     -join      : -leave     : \
     -owner     : -request   : \
     -subscribe : -unsubscribe
  transport = mailman3_transport

# /etc/exim4/conf.d/transport/55_mm3_transport
mailman3_transport:
  driver = smtp
  protocol = lmtp
  allow_localhost
  hosts = localhost
  port = MM3_LMTP_PORT
  rcpt_include_affixes = true

Troubleshooting

The most likely causes of failure to deliver to Mailman are typos in the configuration, and errors in the MM3_HOME macro or the mm_domains list. Mismatches in the LMTP port could be a cause. Finally, Exim’s router configuration is order-sensitive. Especially if you are being tricky and supporting Mailman 2 and Mailman 3 at the same time, you could have one shadow the other.

Exim 4 documentation

There is copious documentation for Exim. The parts most relevant to configuring communication with Mailman 3 are the chapters on the accept router and the LMTP transport. Unless you are already familiar with Exim configuration, you probably want to start with the chapter on how Exim receives and delivers mail.

qmail

qmail is a MTA written by djb and, though old and not updated, still bulletproof and occassionally in use.

Mailman settings

Mostly defaults in mailman.cfg:

[mta]
# NullMTA is just implementing the interface and thus satisfying Mailman
# without doing anything fancy
incoming: mailman.mta.null.NullMTA
# Mailman should not be run as root.
# Use any convenient port > 1024.  8024 is a convention, but can be
# changed if there is a conflict with other software using that port.
lmtp_port: 8024

This will listen on localhost:8024 with LMTP and deliver outgoing messages to localhost:25. See mailman/config/schema.cfg for more information on these settings.

qmail configuration

It is assumed that qmail is configured to use the .qmail* files in a user’s home directory, however the instructions should easily be adaptable to other qmail configurations. However, it is required that Mailman has a (sub)domain respectively a namespace on its own. A helper script called qmail-lmtp is needed and can be found in the contrib/ directory of the Mailman source tree and assumed to be on $PATH here.

As qmail puts every namespace in the address, we have to filter it out again. If your main domain is example.com and you assign lists.example.com to the user mailman, qmail will give you the destination address mailman-spam@lists.example.com while it should actually be spam@lists.example.com. The second argument to qmail-lmtp defines how many parts (separated by dashes) to filter out. The first argument specifies the LMTP port of Mailman. Long story short, as user mailman:

% chmod +t "$HOME"
% echo '|qmail-lmtp 8042 1' > .qmail # put appropriate values here
% ln -sf .qmail .qmail-default
% chmod -t "$HOME"

Sendmail

The core Mailman developers generally do not use Sendmail, so experience is limited. Any and all contributions are welcome! The follow information from a post by Gary Algier <gaa@ulticom.com> may be useful as a starting point, although it describes Mailman 2:

I have it working fine. I recently replaced a very old implementation of sendmail and Mailman 2 on Solaris with a new one on CentOS 6. When I did so, I used the POSTFIX_ALIAS_CMD mechanism to automatically process the aliases. See:

https://mail.python.org/pipermail/mailman-users/2004-June/037518.html

In mm_cfg.py:

MTA='Postfix'
POSTFIX_ALIAS_CMD = '/usr/bin/sudo /etc/mail/import-mailman-aliases'

/etc/mail/import-mailman-aliases contains:

#! /bin/sh
/bin/cp /etc/mailman/aliases /etc/mail/mailman.aliases
/usr/bin/newaliases

In /etc/sudoers.d/mailman:

Cmnd_Alias IMPORT_MAILMAN_ALIASES = /etc/mail/import-mailman-aliases
apache ALL= NOPASSWD: IMPORT_MAILMAN_ALIASES
mailman ALL= NOPASSWD: IMPORT_MAILMAN_ALIASES
Defaults!IMPORT_MAILMAN_ALIASES !requiretty

In the sendmail.mc file I changed:

define(`ALIAS_FILE', `/etc/aliases')dnl

to:

define(`ALIAS_FILE', `/etc/aliases,/etc/mail/mailman.aliases')dnl

so that the Mailman aliases would be in a separate file.

The main issue here is that Mailman 2 expects to receive messages from the MTA via pipes, whereas Mailman 3 uses LMTP exclusively. Recent Sendmail does support LMTP, so it’s a matter of configuring a stock Sendmail. But rather than using aliases, it needs to be configured to relay to the LMTP port of Mailman.