Exim+Dovecot+Sieve via LMTP

* * * The guide is already outdated as of November 2018,
and still the guide can be used on servers without Directadmin or for your personal purposes.
Check the link for other similar guides* * *

To switch Exim+Dovecot+Sieve to LMTP from LDA one need to apply some changes.

LMTP is designed as an alternative to normal SMTP for situations where the receiving side does not have a mail queue, such as a mail storage server acting as a Mail Delivery Agent. Mail queues are an inherent requisite of SMTP. In situations in which mail queues are not possible, LMTP is desirable, since a mail storage server should manage only its mail store without having to allocate more storage for a mail queue.

You can configure LMTP on TCP or UNIX sockets (just add one of the following sections into /etc/dovecot.conf): 

service lmtp {
   inet_listener lmtp {
      address = 127.0.0.1
      port = 24024
   }
}

or

service lmtp {
   unix_listener lmtp {
      mode = 0666
   }
}

if you want to use UNIX sockets (see http://wiki2.dovecot.org/LMTP for more details) and restart dovecot.

If you've chosen TCP, then check netstat output the following way:

# netstat -antpl | grep 24024
tcp        0      0 127.0.0.1:24024             0.0.0.0:*                   LISTEN      13744/dovecot

If you've chosen sockets, then you can check existence of /var/run/dovecot/lmtp the following way:

# stat /var/run/dovecot/lmtp
  File: `/var/run/dovecot/lmtp'
  Size: 0               Blocks: 0          IO Block: 4096   socket
Device: 32h/50d Inode: 58690646    Links: 1
Access: (0666/srw-rw-rw-)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2013-04-03 20:13:32.000000000 +0700
Modify: 2013-04-03 20:13:32.000000000 +0700
Change: 2013-04-03 20:13:32.000000000 +0700

Don't know what for, but you can denote the both variants in /etc/dovecot.conf

Now you should update your /etc/exim.conf: Find a section in /etc/exim.conf

address_reply:
  driver = autoreply

and after that add the following lines:

dovecot_lmtp_tcp:
   driver = smtp
   protocol = lmtp
   port = 24024
   hosts = localhost
   allow_localhost
   return_path_add

dovecot_lmtp_udp:
   driver = lmtp
   socket = /var/run/dovecot/lmtp
   return_path_add

Depending on what you're going to use you should change

transport = virtual_localdelivery

either to

transport = dovecot_lmtp_tcp

for TCP, or to

transport = dovecot_lmtp_udp

for sockets in section

virtual_user:

of /etc/exim.conf And restart exim.

p.s. UNIX sockets should be a quicker way to use the things.