How To Install and Configure Postfix on Ubuntu 20.04

A previous version of this tutorial was written by Justin Ellingwood

Introduction

Postfix is a popular open-source Mail Transfer Agent (MTA) that can be used to route and deliver email on a Linux system. It is estimated that around 25% of public mail servers on the internet run Postfix.

In this guide, you’ll learn how to install and configure Postfix on an Ubuntu 20.04 server. Then, you’ll test that Postfix is able to correctly route mail by installing s-nail, a Mail User Agent (MUA), also known as an email client.

Note that the goal of this tutorial is to help you get Postfix up and running quickly with only some bare-bones email functionality. You won’t have a full featured email server by the end of this guide, but you will have some of the foundational components of such a setup to help you get started.

Prerequisites

In order to follow this guide, you’ll need the following:

• A server running Ubuntu 20.04 to function as your Postfix mail server. This server should have a non-root user with sudo privileges and a firewall configured with UFW. You can follow our Ubuntu 20.04 initial server setup guide to set this up.
• A Fully Qualified Domain Name pointed at your Ubuntu 20.04 server. You can find help on setting up your domain name with DigitalOcean by following our Domains and DNS Networking documentation. Be aware that if you plan on accessing mail from an external location, you will need to make sure you have an MX record pointing to your mail server as well.

Note that this tutorial assumes that you are configuring a host that has the FQDN of mail.example.com. Wherever necessary, be sure to change example.com or mail.example.com to reflect your own FQDN.

Step 1 — Installing Postfix

Postfix is included in Ubuntu’s default repositories, so you can install it with APT.

To begin, update your local apt package cache:

sudo apt update

Then install the postfix package with the following command. Note that here we pass the DEBIAN_PRIORITY=low environmental variable into this installation command. This will cause the installation process to prompt you to configure some additional options:

sudo DEBIAN_PRIORITY=low apt install postfix

This installation process will open a series of interactive prompts. For the purposes of this tutorial, use the following information to fill in your prompts:

• General type of mail configuration?: For this, choose Internet Site since this matches our infrastructure needs.
• System mail name: This is the base domain used to construct a valid email address when only the account portion of the address is given. For instance, let’s say the hostname of your server is mail.example.com. You will likely want to set the system mail name to example.com so that, given the username user1, Postfix will use the address user1@example.com.
• Root and postmaster mail recipient: This is the Linux account that will be forwarded mail addressed to root@ and postmaster@. Use your primary account for this. In this example case, sammy.
• Other destinations to accept mail for: This defines the mail destinations that this Postfix instance will accept. If you need to add any other domains that this server will be responsible for receiving, add those here. Otherwise, the default will be sufficient.
• Force synchronous updates on mail queue?: Since you are likely using a journaled filesystem, accept No here.
• Local networks: This is a list of the networks for which your mail server is configured to relay messages. The default will work for most scenarios. If you choose to modify it, though, make sure to be very restrictive in regards to the network range.
• Mailbox size limit: This can be used to limit the size of messages. Setting it to 0 disables any size restriction.
• Local address extension character: This is the character that can be used to separate the regular portion of the address from an extension (used to create dynamic aliases). The default, + will work for this tutorial.
• Internet protocols to use: Choose whether to restrict the IP version that Postfix supports. For the purposes of this tutorial, pick all.

To be explicit, these are the settings used in this guide:

• General type of mail configuration?: Internet Site
• System mail name: example.com (not mail.example.com)
• Root and postmaster mail recipient: The username of your primary Linux account (sammy in our examples)
• Other destinations to accept mail for: $myhostname, example.com, mail.example.com, localhost.example.com, localhost
• Force synchronous updates on mail queue?: No
• Local networks: 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
• Mailbox size limit: 0
• Local address extension character: +
• Internet protocols to use: all

Note: If you need to ever return to change these settings, you can do so by typing:

sudo dpkg-reconfigure postfix

The prompts will be pre-populated with your previous responses.

When the installation process finishes, you’re ready to make a few updates to your Postfix configuration.

Step 2 — Changing the Postfix Configuration

Now you can adjust some settings that the package installation process didn’t prompt you for. Many of Postfix’s configuration settings are defined in the /etc/postfix/main.cf file. Rather than editing this file directly, you can use Postfix’s postconf command to query or set configuration settings.

To begin, set the location for your non-root Ubuntu user’s mailbox. In this guide, we’ll use the Maildir format, which separates messages into individual files that are then moved between directories based on user action. The alternative option that isn’t covered in this guide is the mbox format, which stores all messages within a single file.

Set the home_mailbox variable to Maildir/. Later, you will create a directory structure under that name within your user’s home directory. Configure home_mailbox by typing:

sudo postconf -e 'home_mailbox= Maildir/'

Next, set the location of the virtual_alias_maps table, which maps arbitrary email accounts to Linux system accounts. Run the following command, which maps the table location to a hash database file named /etc/postfix/virtual:

sudo postconf -e 'virtual_alias_maps= hash:/etc/postfix/virtual'

Now that you’ve defined the location of the virtual maps file in your main.cf file, you can create the file itself and begin mapping email accounts to user accounts on your Linux system. Create the file with your preferred text editor; in this example, we’ll use nano:

sudo nano /etc/postfix/virtual

List any addresses that you wish to accept email for, followed by a whitespace and the Linux user you’d like that mail delivered to.

For example, if you would like to accept email at contact@example.com and admin@example.com and would like to have those emails delivered to the sammy Linux user, you could set up your file like this:

contact@example.com sammy
admin@example.com sammy

After you’ve mapped all of the addresses to the appropriate server accounts, save and close the file. If you used nano, do this by pressing CTRL + X, Y, then ENTER.

Apply the mapping by typing:

sudo postmap /etc/postfix/virtual

Restart the Postfix process to be sure that all of your changes have been applied:

sudo systemctl restart postfix

Assuming you followed the prerequisite Initial Server Setup guide, you will have configured a firewall with UFW. This firewall will block external connections to services on your server by default unless those connections are explicitly allowed, so you’ll have to add a firewall rule to allow an exception for Postfix.

You can allow connections to the service by typing:

sudo ufw allow Postfix

With that, Postfix is configured and ready to accept external connections. However, you aren’t yet ready to test it out with a mail client. Before you can install a client and use it to interact with the mail being delivered to your server, you’ll need to make a few changes to your Ubuntu server’s setup.

Step 3 — Installing the Mail Client and Initializing the Maildir Structure

In order to interact with the mail being delivered, this step will walk you through the process of installing the s-nail package. This is a feature-rich variant of the BSD xmail client which can handle the Maildir format correctly.

Before installing the client, though, it would be prudent to make sure your MAIL environment variable is set correctly. s-nail will look for this variable to figure out where to find mail for your user.

To ensure that the MAIL variable is set regardless of how you access your account — whether through ssh, su, su -, or sudo, for example — you’ll need to set the variable in the /etc/bash.bashrc file and add it to a file within /etc/profile.d to make sure it is set for all users by default.

To add the variable to these files, type:

echo 'export MAIL=~/Maildir' | sudo tee -a /etc/bash.bashrc | sudo tee -a /etc/profile.d/mail.sh

To read the variable into your current session, source the /etc/profile.d/mail.sh file:

source /etc/profile.d/mail.sh

With that complete, install the s-nail email client with APT:

sudo apt install s-nail

Before running the client, there are a few settings you need to adjust. Open the /etc/s-nail.rc file in your editor:

sudo nano /etc/s-nail.rc

At the bottom of the file, add the following options:

. . .
set emptystart
set folder=Maildir
set record=+sent

Here’s what these lines do:

• set emptystart: allows the client to open even with an empty inbox
• set folder=Maildir: sets the Maildir directory to the internal folder variable
• set record=+sent creates a sent mbox file for storing sent mail within whichever directory is set as the folder variable, in this case Maildir

Save and close the file when you are finished. You’re now ready to initialize your system’s Maildir structure.

A quick way to create the Maildir structure within your home directory is to send yourself an email with the s-nail command. Because the sent file will only be available once the Maildir is created, you should disable writing to it for this initial email. Do this by passing the -Snorecord option.

Send the email by piping a string to the s-nail command. Adjust the command to mark your Linux user as the recipient:

echo 'init' | s-nail -s 'init' -Snorecord sammy

Note: You may get the following response:

Output
Can't canonicalize "/home/sammy/Maildir"

This is normal and may only appear when sending this first message.

You can can check to make sure the directory was created by looking for your ~/Maildir directory:

ls -R ~/Maildir

You will see the directory structure has been created and that a new message file is in the ~/Maildir/new directory:

Output
/home/sammy/Maildir/:
cur  new  tmp

/home/sammy/Maildir/cur:

/home/sammy/Maildir/new:
1463177269.Vfd01I40e4dM691221.mail.example.com

/home/sammy/Maildir/tmp:

Now that the directory structure has been created, you’re ready to test out the s-nail client by viewing the init message you sent and sending a message to an external email address.

Step 5 — Testing the Client

To open the client, run the s-nail command:

s-nail

In your console, you’ll see a rudimentary inbox with the init message waiting:

Output
s-nail version v14.9.15.  Type `?' for help
"/home/sammy/Maildir": 1 message 1 new
>N  1 [email protected]     2020-05-19 15:40   14/392   init

Press ENTER to display the message:

Output
[-- Message  1 -- 14 lines, 369 bytes --]:
From [email protected] Tue May 19 15:40:48 2020
Date: Tue, 19 May 2020 15:40:48 +0000
To: [email protected]
Subject: init
Message-Id: <[email protected]>
From: [email protected]

init

You can get back to the message list by typing h, and then ENTER:

h

Output
>R  1 [email protected]     2020-05-19 15:40   14/392   init 

Notice that the message now has a state of R, indicating that it’s been read.

Since this message isn’t very useful, you can delete it by pressing d, and then ENTER:

d

To get back to the terminal, type q and then ENTER:

q

As a final test, check whether s-nail is able to correctly send email messages. To do this, you can pipe the contents of a text file into the s-nail process, like you did with the init message you sent in the previous step.

Begin by writing a test message in a text editor:

nano ~/test_message

Inside, enter some text you’d like to send:

Hello,

This is a test.  Please confirm receipt!

Save and close the file after writing your message.

Then, use the cat command to pipe the message to the s-nail process. You can do so with the following example, which uses these options:

• -s: This defines the subject line of the email message
• -r: An optional change to the “From:” field of the email. By default, the Linux user you are logged in as will be used to populate this field. The -r option allows you to override this with a valid address, such as one of those you defined in the /etc/postfix/virtual file. To illustrate, the following command uses [email protected]

Also, be sure to change user@email.com to a valid email address which you have access to:

cat ~/test_message | s-nail -s 'Test email subject line' -r [email protected] user@email.com

Then, navigate to the inbox for the email address to which you sent the message. You will see your message waiting there almost immediately.

Note: If the message isn’t in your inbox, it may have been delivered to your Spam folder.

You can view your sent messages within your s-nail client. Start the interactive client again:

s-nail

From the email client, view your sent messages by typing:

file +sent

You’ll see output like this:

Output
+[/home/sammy/Maildir/]sent: 1 message 1 new
▸N  1 [email protected] 2020-05-19 15:47   12/297   Test email subject line 

You can manage sent mail using the same commands you use for incoming mail.

Conclusion

You now have Postfix configured on your Ubuntu 20.04 server. Managing email servers can be a tough task for new system administrators, but with this configuration, you should have enough MTA email functionality to get yourself started.