LMTP in Mailman 3

RFC 2033, Local Mail Transport Protocol (LMTP) provides Mailman with a unique opportunity to provide a better user experience when accepting initial postings. Two improvements in the process are available with LMTP.

First, we can eliminate most of the integration cruft we currently have with supporting multiple MTAs for incoming mail. Most of the major SMTP servers support LMTP delivery, so by providing an LMTP server in Mailman, the hope is that we can avoid all the crufty MTA-specific alias hackery.

Second, and perhaps more importantly, we can support better anti-backscatter and anti-spam defenses for messages sent to the mailing lists, by rejecting messages in the SMTP dialog instead of having to make the determination way later and sending a bounce message. Ian Eiloart describes what is possible.

How to set up LMTP delivery for Mailman 3

Here is detailed information about how Mailman should integrate with the various MTAs using LMTP.

Postfix

Postfix uses the Postfix lmtp client to transport messages to a LMTP
server. You probably already have this in your Postfix master.cf file:

# ==========================================================================
# service type  private unpriv  chroot  wakeup  maxproc command + args
#               (yes)   (yes)   (yes)   (never) (100)
# ==========================================================================
...
lmtp      unix  -       -       -       -       -       lmtp
...

We're going to use a transport map to tell Postfix to deliver all messages
destined for a Mailman mailing list to its lmtp service. Fortunately, Mailman
is already able to write the correct transport map based on your configuration
settings. All you need to do is add a transport_maps setting in your
Postfix main.cf file:

transport_maps =
    ...
    hash:/path/to/mailman/var/data/postfix_lmtp
    ...

Now Postfix knows where it should transport messages to, but it doesn't know
yet it should accept messages for the given recipients - it doesn't know the
recipients are valid recipients. You can reuse the Mailman generated
transport map for this by adding the following to your main.cf file:

local_recipient_maps =
    ...
    hash:/path/to/mailman/var/data/postfix_lmtp
    ...

Virtual domain

If the mailman list addresses are part of $virtual_alias_domains or $virtual_mailbox_domains add postfix_lmtp to the listing of $virtual_alias_maps:

virtual_alias_maps =
    ...
    hash:/path/to/mailman/var/data/postfix_lmtp
    ...

Relay domain

If the mailman list addresses are part of the relay domain namespace add postfix_lmtp to the listing of $relay_recipient_maps:

relay_recipient_maps =
    ...
    hash:/path/to/mailman/var/data/postfix_lmtp
    ...

Once Postfix has been reloaded the new settings will take effect.

A dedicated list server

For a dedicated list server e.g. list.example.com simply add the hostname as key to a transport map and specify the mailman LMTP server as destination:

# key                           value
list.example.org                lmtp:inet:localhost

Be aware though that Postfix has no knowledge of valid recipients in the destination's (sub)domain (here: list.example.org) if you specify the list server as noted above. Postfix will accept and transport any message destined for list.example.org to the mailman LMTP server and it will be the LMTP servers task to reject messages for invalid or non-existing recipients.

There are two ways to prevent putting such load on the mailman LMTP server. First specify each valid recipient address in a transport table as shown in the initial example. Alternatively use the reject_unverified_recipient option and let the Postfix verify(8) daemon find out if the recipient address exists. This way Postfix will reject messages to non-exsisting recipients during the SMTP session with the client that attempts to deliver the message. See ADDRESS_VERIFICATION_README for details on implementing the reject_unverified_recipient option.

A dedicated lmtp client

There may be situations where a dedicated lmtp client, that differs in its configuration from the default lmtp client settings, is required.

To create such a client add a new service (here: mailman3) to the Postfix master.cf configuration file and add the configuration options which should override the default lmtp client behaviour:

# ==========================================================================
# service type  private unpriv  chroot  wakeup  maxproc command + args
#               (yes)   (yes)   (yes)   (never) (100)
# ==========================================================================
...
mailman3   unix  -       -       -       -       -       lmtp
    -o lmtp_send_xforward_command=yes
    -o disable_dns_lookups=yes
...

Then specify the new service in the transport table e.g. /etc/postfix/mailman_lists:

# key                           value
mailman@example.org             mailman3:inet:localhost
mailman-admin@example.org       mailman3:inet:localhost
mailman-bounces@example.org     mailman3:inet:localhost
mailman-confirm@example.org     mailman3:inet:localhost
...

Exim

There are two variants of LMTP support in Exim transports.  The first of these exists as an option to the SMTP driver, instructing it to talk LMTP over the SMTP connection.  Such a transport will look like this:

mailman_remote_lmtp:
    driver = smtp
    protocol = lmtp
    hosts = localhost
    allow_localhost 

There is an independant LMTP transport driver, which is able to communicate using unix sockets.  For reference, this transport will look like the following, excluding any additional local configuration options:

mailman_local_lmtp:
    driver = lmtp
    socket = "/var/run/path/to/unix/socket"
    batch_max = 40
    user = mailman

Email routing using Exim

The exim router should only need its transport option changed, the rest of the logic can remain the same.  One can consult the output of genaliases:

mailman_aliases:
    driver = accept
    domains = +local_domains
    condition = ${lookup${local_part}lsearch{/path/to/genaliases/output}}
    transport = mailman_local_lmtp

Alternatively, one can check directly for a list's existence:

 mailman_direct:
    driver = accept
    domains = +local_domains
    require_files = /mail/mailman/lists/${lc::$local_part}/config.pck
    local_part_suffix_optional
    local_part_suffix =  -admin     : \
                      -bounces   : -bounces+* : \
                      -confirm   : -confirm+* : \
                      -join      : -leave     : \
                      -owner     : -request   : \
                      -subscribe : -unsubscribe
    transport = mailman_local_lmtp

Access Controls in Exim

Once you've set up routing and transports, you can use Exim's call forwards in an ACL (with the use_sender option) to determine whether the sender is permitted to post to the list. You should get a definitive answer, and this mechanism allows you to use a remote Mailman installation as if it were local - that is, you don't need to consult any local files. See section 40.40 of the Exim docs.

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