Postfix and Dovecot LMTP

Basic Configuration

The first step is to enable LMTP in dovecot.conf:

protocols = imap lmtp

Socket Configuration

The LMTP service can be bound to both INET or Unix sockets.

In this example, a Unix socket is placed inside the Postfix spool with appropriate permissions set:

service lmtp {
  unix_listener /var/spool/postfix/private/dovecot-lmtp {
    group = postfix
    mode = 0600
    user = postfix
  }
}

Note that the socket needs to be placed there because Postfix access is limited to this directory.

Plugin Support

Plugin support can be enabled at protocol level for quota plugin and sieve plugin.

protocol lmtp {
  # REQUIRED
  postmaster_address = postmaster@domainname
  mail_plugins = quota sieve
}

Postfix main.cf Configuration

The final step is to tell Postfix to use this socket for final delivery, in this case of a virtual user scenario:

virtual_transport = lmtp:unix:private/dovecot-lmtp

For a non virtual user setup (i.e., when mail_location = maildir:~/.maildir):

mailbox_transport = lmtp:unix:private/dovecot-lmtp

Dynamic Address Verification with LMTP

You can use LMTP and the Postfix setting "reject_unverified_recipient" for dynamic address verification. It's useful as Postfix doesn't need to query an external datasource (MySQL, LDAP...). Postfix maintain a local database with existing/non-existing addresses (you can configure how long positive/negative results should be cached).

See: Postfix reject_unverified_recipient.

To use LMTP and dynamic address verification, you must first get Dovecot working. Then you can configure Postfix to use LMTP and set "reject_unverified_recipient" in the smtpd_recipient_restrictions.

On every incoming email, Postfix will probe if the recipient address exists. You will see entries in your logfile:

Recipient address rejected: undeliverable address: host tux.example.com[private/dovecot-lmtp] said: 550 5.1.1 < tzknvtr@example.com > User doesn't exist: tzknvtr@example.com (in reply to RCPT TO command); from=< cnrilrgfclra@spammer.org > to=< tzknvtr@example.com >

If the recipient address exists (status=deliverable) Postfix accepts the mail.

Note

You cannot use "reject_unverified_recipient" with "pipe", so this doesn't work with the Dovecot LDA "deliver".