Dovecot LMTP and Exim

Exim provides support for LMTP over UNIX sockets using the LMTP transport, your distribution may/not provide this, run exim -bV and check for ‘lmtp’ in ‘Transports:’. Support for LMTP over TCP sockets is provided by the SMTP transport.

Using LMTP over UNIX Socket

Use this configuration if dovecot runs on the same host as exim.

Example router:

local_user:
        debug_print = "R: local_user for $local_part@$domain"
        driver = accept
        domains = +local_domains
        check_local_user
        transport = dovecot_lmtp
        cannot_route_message = Unknown user

Example transport:

dovecot_lmtp:
        driver = lmtp
        socket = /var/run/dovecot/lmtp
        #maximum number of deliveries per batch, default 1
        batch_max = 200
        #allow suffixes/prefixes (default unset)
        rcpt_include_affixes

Using LMTP over TCP Socket

Example router:

local_user:
        transport = dovecot_lmtp
        domains = +local_domains
        driver = manualroute
        route_list = "* 192.168.1.0 byname"
        #if destination server is the local host enable this
        #self = send

Set IP and port as appropriate to your setup.

Example transport:

dovecot_lmtp:
        driver = smtp
        #allow suffixes/prefixes (default unset)
        rcpt_include_affixes
        protocol = lmtp
        port = 2525

Striping domain to avoid user unknown / doesn’t exist error

If you are using a userdb which does not have domain names, you may need to add a setting to 20-lmtp.conf

protocol lmtp {
   ...
   # use %n to strip away the domain part
   auth_username_format = %n
}

Symptoms:

• Exim says something like “LMTP error after RCPT … 550 … User doesn’t exist user@domain”

• Dovecot verbose log says something like “auth-worker(9048): passwd(user@domain): unknown user”

Verifying recipients using LMTP

You can use callout verification to avoid accepting mail for addresses which do not exist in Dovecot. Below is a config snippet which could be used in acl_smtp_rcpt to achieve this:

deny
        message = invalid recipient
        domains = +local_domains
        !verify = recipient/callout=no_cache

For more information on address verification see the related section of the Exim specification.

Delivering mails case insensitively

Warning:Just use this setup if all your login names contain only lower case characters! (On Linux see /etc/adduser.conf under NAME_REGEX variable).

Exim retains the case of the local part. Dovecot’s LMTP may fail looking up an incorrect cased local part in your userdb. You can solve this problem by extending the protocol lmtp section:

protocol lmtp {
   ...
   # use %Ln to strip away the domain part
   auth_username_format = %Lu
}

(If you don’t mind allowing case insensitive logins for dovecot authentication, you may set auth_username_format in the global configuration accordingly and renounce the above change).

In case you prefer to configure exim to lower case the local part instead, add a router just before your local delivery router:

lowercase_local:
        debug_print = "R: lower case local_part for local delivery"
        driver = redirect
        redirect_router = local_user
        data = ${lc:${local_part}}

Make sure to reference the name you have chosen for your local delivery router within redirect_router.