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Set up Postfix for incoming email (FREE SELF)

This document will take you through the steps of setting up a basic Postfix mail server with IMAP authentication on Ubuntu, to be used with incoming email.

The instructions make the assumption that you are using the email address incoming@gitlab.example.com, that is, username incoming on host gitlab.example.com. Don't forget to change it to your actual host when executing the example code snippets.

Configure your server firewall

Open up port 25 on your server so that people can send email into the server over SMTP.
If the mail server is different from the server running GitLab, open up port 143 on your server so that GitLab can read email from the server over IMAP.

Install packages

Install the postfix package if it is not installed already:
```
sudo apt-get install postfix
```
When asked about the environment, select 'Internet Site'. When asked to confirm the hostname, make sure it matches gitlab.example.com.
Install the mailutils package.
```
sudo apt-get install mailutils
```

Create user

Create a user for incoming email.
```
sudo useradd -m -s /bin/bash incoming
```
Set a password for this user.
```
sudo passwd incoming
```
Be sure not to forget this, you'll need it later.

Test the out-of-the-box setup

Connect to the local SMTP server:

telnet localhost 25

You should see a prompt like this:

Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 gitlab.example.com ESMTP Postfix (Ubuntu)

If you get a Connection refused error instead, verify that postfix is running:

sudo postfix status

If it is not, start it:

sudo postfix start

Send the new incoming user an email to test SMTP, by entering the following into the SMTP prompt:
```
ehlo localhost
mail from: root@localhost
rcpt to: incoming@localhost
data
Subject: Re: Some issue

Sounds good!
.
quit
```
NOTE: The . is a literal period on its own line.

If you receive an error after entering rcpt to: incoming@localhost then your Postfix my_network configuration is not correct. The error will say 'Temporary lookup failure'. See Configure Postfix to receive email from the Internet._

Check if the incoming user received the email:

su - incoming
mail

You should see output like this:

"/var/mail/incoming": 1 message 1 unread
>U   1 root@localhost                           59/2842  Re: Some issue

Quit the mail app:

Sign out of the incoming account, and go back to being root:
```
logout
```

Configure Postfix to use Maildir-style mailboxes

Courier, which we install later to add IMAP authentication, requires mailboxes to have the Maildir format, rather than mbox.

Configure Postfix to use Maildir-style mailboxes:
```
sudo postconf -e "home_mailbox = Maildir/"
```
Restart Postfix:
```
sudo /etc/init.d/postfix restart
```
Test the new setup:
1. Follow steps 1 and 2 of Test the out-of-the-box setup.
2. Check if the incoming user received the email:
```
su - incoming
MAIL=/home/incoming/Maildir
mail
```
  You should see output like this:
```
"/home/incoming/Maildir": 1 message 1 unread
>U   1 root@localhost                           59/2842  Re: Some issue
```
  Quit the mail app:
```
q
```
If mail returns an error Maildir: Is a directory then your version of mail doesn't support Maildir style mailboxes. Install heirloom-mailx by running sudo apt-get install heirloom-mailx. Then, try the above steps again, substituting heirloom-mailx for the mail command.
Sign out of the incoming account, and go back to being root:
```
logout
```

Install the Courier IMAP server

Install the courier-imap package:

sudo apt-get install courier-imap

And start imapd:

imapd start

The courier-authdaemon isn't started after installation. Without it, IMAP authentication fails:
```
sudo service courier-authdaemon start
```
You can also configure courier-authdaemon to start on boot:
```
sudo systemctl enable courier-authdaemon
```

Configure Postfix to receive email from the internet

Let Postfix know about the domains that it should consider local:

sudo postconf -e "mydestination = gitlab.example.com, localhost.localdomain, localhost"

Let Postfix know about the IPs that it should consider part of the LAN:

Let's assume 192.168.1.0/24 is your local LAN. You can safely skip this step if you don't have other machines in the same local network.
```
sudo postconf -e "mynetworks = 127.0.0.0/8, 192.168.1.0/24"
```
Configure Postfix to receive mail on all interfaces, which includes the internet:
```
sudo postconf -e "inet_interfaces = all"
```
Configure Postfix to use the + delimiter for sub-addressing:
```
sudo postconf -e "recipient_delimiter = +"
```
Restart Postfix:
```
sudo service postfix restart
```

Test the final setup

Test SMTP under the new setup:

Connect to the SMTP server:
```
telnet gitlab.example.com 25
```
You should see a prompt like this:
```
Trying 123.123.123.123...
Connected to gitlab.example.com.
Escape character is '^]'.
220 gitlab.example.com ESMTP Postfix (Ubuntu)
```
If you get a Connection refused error instead, make sure your firewall is set up to allow inbound traffic on port 25.

Send the incoming user an email to test SMTP, by entering the following into the SMTP prompt:

ehlo gitlab.example.com
mail from: root@gitlab.example.com
rcpt to: incoming@gitlab.example.com
data
Subject: Re: Some issue

Sounds good!
.
quit

NOTE: The . is a literal period on its own line.