---
title: LMTP
lastUpdated: 2025-11-01
slug: ispmail-trixie/lmtp
sidebar:
  order: 167
---

import { Aside } from "@astrojs/starlight/components";
import StepListReceive from "../../../components/StepListReceive.astro";

<StepListReceive currentStep={5} />

Glad to see that you are still with me. We are very close to receiving our first email. If you feel lost, please review
the [slideshow](/ispmail-trixie/overview/) from earlier in this guide.

As explained in the previous section, Postfix speaks SMTP and receives the email from the internet. Postfix could even
save the email to a mailbox on disk. But instead we will use Dovecot for the final delivery. Actually Dovecot's main
purpose is to let users fetch their email using the IMAP protocol. But it provides additional features we can use as
well.

So we need tell Postfix to hand over the incoming email to Dovecot. The communication between Postfix and Dovecot will
happens using LMTP – the [local mail transfer protocol](https://en.wikipedia.org/wiki/Local_Mail_Transfer_Protocol).
LMTP is a lightweight variant of SMTP. It is meant for email communication between internal services that trust each
other.

## Dovecot listens to LMTP

First we need to add a UNIX socket where Dovecot listens for incoming LMTP connections:

```sh title="Run this on your server"
cat > /etc/dovecot/conf.d/99-ispmail-lmtp-listener.conf << EOF
service lmtp {
  # Used internally by Dovecot
  unix_listener lmtp {
  }

  # Listen to LMTP connections from Postfix
  unix_listener /var/spool/postfix/private/dovecot-lmtp {
    mode = 0600
    user = postfix
    group = postfix
  }
}
EOF
```

The first `unix_listener lmtp` is just used internally by Dovecot and does not concern us. The second part is what you
will be adding. It makes Dovecot create a socket file at `/var/spool/postfix/private/dovecot-lmtp`. As Postfix has its
home in `/var/spool/postfix` it can access that socket to speak LMTP with Dovecot.

A socket is similar to a TCP port but it's only available for other processes running on same system. That's why you
also set the access mode, user and group so that Postfix can actually use it.

## Postfix talks to Dovecot using LMTP

The “_virtual_transport_" in Postfix defines the service to use for delivering emails to the local system. Dovecot has
created a socket file and is ready to listen to incoming LMTP connections. We just need to tell Postfix to send emails
there. So please run:

```sh title="Run this on your server"
postconf virtual_transport=lmtp:unix:private/dovecot-lmtp
```

The syntax looks crazy, but it’s actually simple. You just told Postfix to use the LMTP protocol. And that we want to
use a UNIX socket on the same system (instead of a TCP connection). And the socket file is located at
`/var/spool/postfix/private/dovecot-lmtp` – because `/var/spool/postfix` is the Postfix's home directory.

## Send a test mail locally

<StepListReceive currentStep={6} />

Finally we are ready for our first email. Using the `swaks` command you can create a test email to `john@example.org`
and send it to Postfix:

```sh title="Run this on your server"
swaks --server localhost --to john@example.org
```

If all goes as planned, you will see an SMTP dialog like this:

```
=== Trying localhost:25...
=== Connected to localhost.
<-  220 mail ESMTP Postfix (Debian)
 -> EHLO mail
<-  250-mail
<-  250-PIPELINING
<-  250-SIZE 10240000
<-  250-VRFY
<-  250-ETRN
<-  250-STARTTLS
<-  250-ENHANCEDSTATUSCODES
<-  250-8BITMIME
<-  250-DSN
<-  250-SMTPUTF8
<-  250 CHUNKING
 -> MAIL FROM:<root@mail>
<-  250 2.1.0 Ok
 -> RCPT TO:<john@example.org>
<-  250 2.1.5 Ok
 -> DATA
<-  354 End data with <CR><LF>.<CR><LF>
 -> Date: Sun, 24 Aug 2025 21:08:59 +0000
 -> To: john@example.org
 -> From: root@mail
 -> Subject: test Sun, 24 Aug 2025 21:08:59 +0000
 -> Message-Id: <20250824210859.021444@mail>
 -> X-Mailer: swaks v20240103.0 jetmore.org/john/code/swaks/
 ->
 -> This is a test mailing
 ->
 ->
 -> .
<-  250 2.0.0 Ok: queued as 2742B221FA
 -> QUIT
<-  221 2.0.0 Bye
=== Connection closed with remote host.
```

If you see a `250 2.0.0 Ok: queued as…" near the end of this dialog, then Postfix has received the email successfully.

Check if Postfix managed to pass the email to Dovecot:

```sh title="Run this on your server"
journalctl -n 10 -u dovecot
```

The line you are looking for looks like:

```
Aug 24 21:36:22 mail dovecot[23214]: lmtp(john@example.org)<…><…>: msgid=<20250824213622.023221@mail>: saved mail to INBOX
```

Does it say `saved mail to INBOX`? Awesome. The email should be visible in John's home directory. Check it:

```sh title="Run this on your server"
find /var/vmail/example.org/john
```

Does it look like this?

```
/var/vmail/
/var/vmail/example.org
/var/vmail/example.org/john
/var/vmail/example.org/john/Maildir
/var/vmail/example.org/john/Maildir/maildirfolder
/var/vmail/example.org/john/Maildir/new
/var/vmail/example.org/john/Maildir/new/1756071382.M198387P23222.mail,S=635,W=655
/var/vmail/example.org/john/Maildir/cur
/var/vmail/example.org/john/Maildir/tmp
/var/vmail/example.org/john/Maildir/dovecot-uidvalidity.68ab85d6
/var/vmail/example.org/john/Maildir/dovecot.index.log
/var/vmail/example.org/john/Maildir/dovecot-uidvalidity
/var/vmail/example.org/john/Maildir/dovecot.index.cache
/var/vmail/example.org/john/Maildir/dovecot-uidlist
/var/vmail/example.org/john/Maildir/dovecot.list.index.log
```

The important file here is `…/Maildir/new/1756071382.M198387P23222.mail,S=635,W=655`. The numbers will look differently
on your system. But this is the actual mail that got delivered. You can view it with `cat` and will find the test email
you just sent using `swaks`.

Don't worry about the other files like `maildirfolder` or the files that sound like `dovecot…`. Those help Dovecot keep
track of the mailbox. You can safely ignore them.

## Your actual domain

Obviously the previous examples dealt with the `example.org` domain which is not your actual domain. Feel free to add
your own domain to the database and create a test user. The later
[section on managing users](/ispmail-trixie/managing-users-aliases-and-domains/) will explain that in detail.
