Part 2: Install Dovecot IMAP Server on CentOS 8/RHEL 8 & Enable TLS Encryption

This is part 2 of building your own email server from scratch on CentOS 8/RHEL 8 tutorial series. In part 1, we showed you how to set up a basic Postfix SMTP server. In this tutorial, we are going to configure our email server so that we can receive and send emails using a desktop email client like Mozilla Thunderbird or Microsoft Outlook.

To be able to send emails using a desktop email client, we need to enable the submission service in Postfix. To receive emails using a desktop email client, we can install an open-source IMAP server named Dovecot on CentOS 8/RHEL 8 server. And to encrypt our communications, we need a TLS certificate.

Open Ports in Firewall

Run the following command to open email related ports in firewall.

sudo firewall-cmd --permanent --add-service={http,https,smtp-submission,smtps,imap,imaps}

If you use POP3 to fetch emails (I personally don’t), then also add the pop3 and pop3s service.

sudo firewall-cmd --permanent --add-service={pop3,pop3s}

Reload firewalld for the change to take effect.

sudo systemctl reload firewalld

Securing Email Server Traffic with TLS Certificate

When we configure a desktop email client, enabling encryption is always a good idea. We can easily obtain a free TLS certificate from Let’s Encrypt. Issue the following commands to install Let’s Encrypt client (certbot) on CentOS 8/RHEL 8 from the EPEL repository.

sudo dnf install epel-release -y

sudo dnf install certbot

If you don’t have a web server running yet, I recommend you install one (Apache or Nginx), because it’s easier to obtain and install TLS certificate with a web server than using other methods. And in a later tutorial, I will show you how to set up webmail, which requires running a web server.

Note that your CentOS 8/RHEL 8 server might ship with Apache web server by default.

Apache

If you prefer Apache, run the following command to install it.

sudo dnf install httpd

Start Apache and enable auto-start at boot time.

sudo systemctl start httpd

sudo systemctl enable httpd

Install the Certbot Apache plugin.

sudo dnf install python3-certbot-apache

Nginx

If you prefer Nginx, run the following command to install it.

sudo dnf install nginx

Start Nginx and enable auto-start at boot time.

sudo systemctl start nginx

sudo systemctl enable nginx

Install the Certbot Nginx plugin.

sudo dnf install python3-certbot-nginx

Obtaining TLS Certificate with Apache Web Server

You need to have an Apache virtual host for mail.your-domain.com before obtaining Let’s Encrypt TLS certificate. Create the virtual host file:

sudo nano /etc/httpd/conf.d/mail.your-domain.com.conf

Then paste the following text into the file.

<VirtualHost *:80>        
        ServerName mail.your-domain.com

        DocumentRoot /var/www/html/
</VirtualHost>

Save and close the file. Reload Apache for the changes to take effect.

sudo systemctl reload httpd

Once virtual host is created and enabled, run the following command to obtain Let’s Encrypt TLS certificate.

sudo certbot certonly -a -apache --agree-tos --staple-ocsp --email [email protected] -d mail.your-domain.com

After a while, you should see the following lines which means the certificate is successfully obtained. You can also see the directory under which your cert is stored.

Obtaining TLS Certificate with Nginx Web Server

You need to have an Nginx virtual host for mail.your-domain.com before obtaining Let’s Encrypt TLS certificate. Create the virtual host file:

sudo nano /etc/nginx/conf.d/mail.your-domain.com.conf

Next, paste the following text into the file.

server {
      listen 80;
      listen [::]:80;
      server_name mail.your-domain.com;

      root /usr/share/nginx/html/;

      location ~ /.well-known/acme-challenge {
         allow all;
      }
}

Save and close the file. Reload Nginx for the changes to take effect.

sudo systemctl reload nginx

Once virtual host is created and enabled, run the following command to obtain Let’s Encrypt certificate with Nginx plugin.

sudo certbot certonly -a nginx --agree-tos --staple-ocsp --email [email protected] -d mail.your-domain.com

After a while, you should see the following which means the certificate is successfully obtained. You can also see the directory under which your cert is stored.

Enabling Submission Service in Postfix

To send emails from a desktop email client, we need to enable the submission service of Postfix so that the email client can submit emails to Postfix SMTP server. Edit the master.cf file.

sudo nano /etc/postfix/master.cf

In submission section, uncomment or add the following lines. Please allow at least one whitespace (tab or spacebar) before each -o.  In postfix configurations, a preceding whitespace character means that this line is continuation of the previous line. (By default the submission section is commented out. You can copy the following lines and paste them into the file, so you don’t have to manually uncomment or add new text.)

submission     inet     n    -    y    -    -    smtpd
 -o syslog_name=postfix/submission
 -o smtpd_tls_security_level=encrypt
 -o smtpd_tls_wrappermode=no
 -o smtpd_sasl_auth_enable=yes
 -o smtpd_relay_restrictions=permit_sasl_authenticated,reject
 -o smtpd_recipient_restrictions=permit_mynetworks,permit_sasl_authenticated,reject
 -o smtpd_sasl_type=dovecot
 -o smtpd_sasl_path=private/auth

The above configuration enables the submission daemon of Postfix and requires TLS encryption. So later on our desktop email client can connect to the submission daemon in TLS encryption. The submission daemon listens on TCP port 587. STARTTLS is used to encrypt communications between email client and the submission daemon.

Microsoft Outlook only supports submission over port 465. If you are going to use Microsoft outlook mail client, then you also need to enable submission service on port 465 by adding the following lines in the file.

smtps     inet  n       -       y       -       -       smtpd
  -o syslog_name=postfix/smtps
  -o smtpd_tls_wrappermode=yes
  -o smtpd_sasl_auth_enable=yes
  -o smtpd_relay_restrictions=permit_sasl_authenticated,reject
  -o smtpd_recipient_restrictions=permit_mynetworks,permit_sasl_authenticated,reject
  -o smtpd_sasl_type=dovecot
  -o smtpd_sasl_path=private/auth

Save and close the file.

Hint: The SMTP protocol is used when an email client submits emails to an SMTP server.

Next, we need to run the following two commands to specify the location of TLS certificate and private key in Postfix configuration file. Your Let’s Encrypt certificate and private key are stored under /etc/letsencrypt/live/mail.your-domain.com/ directory.

sudo postconf "smtpd_tls_cert_file = /etc/letsencrypt/live/mail.your-domain.com/fullchain.pem"

sudo postconf "smtpd_tls_key_file = /etc/letsencrypt/live/mail.your-domain.com/privkey.pem"

If you want to log TLS connections in the mail log (/var/log/maillog), then run the following two commands.

sudo postconf "smtpd_tls_loglevel = 1"

sudo postconf "smtp_tls_loglevel = 1"

To disable insecure SSL/TLS versions, open the Postfix main configuration file.

sudo nano /etc/postfix/main.cf

Add the following lines at the bottom of the file. (In Nano text editor, you can quickly go to the bottom of a file by pressing Ctrl+W, then Ctrl+V.)

#Force TLSv1.3 or TLSv1.2
smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
smtpd_tls_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
smtp_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
smtp_tls_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1

Save and close the file. Then reload Postfix for the changes to take effect.

sudo systemctl restart postfix

If you run the following command, you will see Postfix is now listening on port 587 and 465.

sudo ss -lnpt | grep master

Installing Dovecot IMAP Server

Enter the following command to install Dovecot on CentOS 8/RHEL 8 server.

sudo dnf install dovecot -y

Check Dovecot version:

dovecot --version

Sample output:

2.3.8 (9df20d2db)

Start Dovecot and enable auto-start at boot time.

sudo systemctl start dovecot

sudo systemctl enable dovecot

Configuring Dovecot

First, edit main config file.

sudo nano /etc/dovecot/dovecot.conf

Add the following line to enable IMAP protocol.

protocols = imap

If you want to use POP3 to fetch emails, then also add the POP3 protocol.

protocols = imap pop3

Note that you should not enable the submission protocol in Dovecot, because we have already enabled submission service in Postfix.

Save and close the file.

Configuring Mailbox Location

mbox is the traditional and default format for storing emails. Each user’s emails are stored in a single file /var/mail/username. You can run the following command to find the mail spool directory.

postconf mail_spool_directory

Sample output:

mail_spool_directory = /var/mail

However, nowadays it’s almost always you want to use the Maildir format to store email messages. The config file for mailbox location is /etc/dovecot/conf.d/10-mail.conf.

sudo nano /etc/dovecot/conf.d/10-mail.conf

Add the following line to use the Maildir format. Email messages will be stored under the Maildir directory under each user’s home directory.

mail_location = maildir:~/Maildir

We also need to add the following line in the file.

mail_privileged_group = mail

Save and close the file. Then add dovecot to the mail group so that Dovecot can read the INBOX.

sudo gpasswd -a dovecot mail

Configuring Authentication Mechanism

Edit the authentication config file.

sudo nano /etc/dovecot/conf.d/10-auth.conf

Uncomment the following line.

disable_plaintext_auth = yes

It will disable plaintext authentication when there’s no SSL/TLS encryption. And if you want to use full email address ([email protected]) to login, add the following line in the file.

auth_username_format = %n

Otherwise, you are able to login with username only (without @your-domain.com). Next, find the following line.

auth_mechanisms = plain

This line only enables the PLAIN authentication mechanism. LOGIN is another authentication mechanism you probably want to add to support older email clients.

auth_mechanisms = plain login

Save and close the file.

Configuring SSL/TLS Encryption

Next, edit SSL/TLS config file.

sudo nano /etc/dovecot/conf.d/10-ssl.conf

You can find the following line, which requires email clients to communicate with Dovecot with TLS encryption.

ssl = required

Then find the following two lines.

ssl_cert = </etc/pki/dovecot/certs/dovecot.pem
ssl_key = </etc/pki/dovecot/private/dovecot.pem

We need to replace values with the location of your SSL/TLS cert and private key. Don’t leave out the < character. It’s necessary.

ssl_cert = </etc/letsencrypt/live/mail.your-domain.com/fullchain.pem
ssl_key = </etc/letsencrypt/live/mail.your-domain.com/privkey.pem

Next, find the following line and uncomment it. (Remove the beginning # character.)

#ssl_dh = </etc/dovecot/dh.pem

Find the following line.

#ssl_min_protocol = TLSv1

This specifies the minimum TLS versions used by Dovecot. TLSv1.0 and TLSv1.1 are insecure. so uncomment this line and change the value to TLSv1.2, which will force Dovecot to use TLSv1.2 or TLSv1.3.

ssl_min_protocol = TLSv1.2

Then find the following line.

#ssl_prefer_server_ciphers = no

It’s a good practice to prefer the server’s order of ciphers over client’s, so uncomment this line and change the value to yes.

ssl_prefer_server_ciphers = yes

Save and close the file. Now we need to generate the Diffie-Hellman parameter with:

sudo openssl dhparam -out /etc/dovecot/dh.pem 4096

If your mail server has a single CPU core, then this is going to take a long time (about 10 minutes). If you can’t wait, you can generate the DH parameters on your local Linux computer, then upload the file to the /etc/dovecot/ directory on the mail server.

SASL Authentication Between Postfix and Dovecot

Edit the following file.

sudo nano /etc/dovecot/conf.d/10-master.conf

Change service auth section to the following so that Postfix can find the Dovecot authentication server. Please be careful about the syntax. Every opening bracket should be terminated by a closing bracket.

service auth {
    unix_listener /var/spool/postfix/private/auth {
      mode = 0600
      user = postfix
      group = postfix
    }
}

Save and close the file.

Auto-create Sent and Trash Folder

Edit the below config file.

sudo nano /etc/dovecot/conf.d/15-mailboxes.conf

To auto-create a folder, simply add the following line in the mailbox section.

auto = create

Example:

 mailbox Trash {
    auto = create
    special_use = Trash
 }

Some common folders you will want to create includes: Drafts, Junk, Trash and Sent. These folders will be created at the user’s home directory. After you save and close all above config files, restart Dovecot.

sudo systemctl restart dovecot

Dovecot will be listening on port 143 (IMAP) and 993 (IMAPS), as can be seen with:

sudo ss -lnpt | grep dovecot

If there’s a configuration error, dovecot will fail to restart, so it’s a good idea to check the status of Dovecot.

systemctl status dovecot

We also need to restart Postfix to allow the LOGIN authentication mechanism.

sudo systemctl restart postfix

Using Dovecot to Deliver Email to Message Store

By default, Postfix uses its builtin local delivery agent (LDA) to move inbound emails to the message store (inbox, sent, trash, Junk, etc). We can configure it to use Dovecot to deliver emails, via the LMTP protocol, which is a simplified version of SMTP. LMTP allows for a highly scalable and reliable mail system. This step is required if you want to use the sieve plugin to filter inbound messages to different folders.

Edit the Dovecot main configuration file.

sudo nano /etc/dovecot/dovecot.conf

Add lmtp to the supported protocols.

protocols = imap lmtp

Save and close the file. Then edit the Dovecot 10-master.conf file.

sudo nano /etc/dovecot/conf.d/10-master.conf

Change the lmtp service definition to the following.

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

Save and close the file. Next, edit the Postfix main configuration file.

sudo nano /etc/postfix/main.cf

Add the following lines at the end of the file. The first line tells Postfix to deliver emails to local message store via the dovecot LMTP server.  The second line disables SMTPUTF8 in Postfix, because Dovecot-LMTP doesn’t support this email extension.

mailbox_transport = lmtp:unix:private/dovecot-lmtp
smtputf8_enable = no

Save and close the file. Finally, restart Postfix and Dovecot.

sudo systemctl restart postfix dovecot

Configure Desktop Email Client

Fire up your desktop email client such as Mozilla Thunderbird and add a mail account. If Thunderbird found your mail server configuration like below, simply click Done button and you will be able to read and send emails.

If Thunderbird didn’t found your mail server configuration, then click Manual config button to enter your mail server details.

• In the incoming server section, select IMAP protocol, enter mail.your-domain.com as the server name, choose port 143 and STARTTLS. Choose normal password as the authentication method.
• In the outgoing section, select SMTP protocol, enter mail.your-domain.com as the server name, choose port 587 and STARTTLS. Choose normal password as the authentication method.

Hint: You can also use port 993 with SSL/TLS encryption for IMAP, and use port 465 with SSL/TLS encryption for SMTP. You should not use port 25 as the SMTP port to submit outgoing emails.

You should now be able to connect to your own email server and also send and receive emails with your desktop email client!

We use local Unix accounts as email addresses, as we did in part 1. For example, if you have a user called user1 on your CentOS/RHEL server, then you have an email address: [email protected], and the password for the email address is the same password for the user1 user. To create a local Unix account, run

sudo adduser user1

Then set a password for this user.

sudo passwd user1

Note: Dovecot doesn’t allow you to login with the root account. You need to create separate user accounts.

You can list all available mailbox users with:

sudo doveadm user '*'

It’s recommended to restart Dovecot after adding users, so Dovecot can recognize new mailbox users.

sudo systemctl restart dovecot

Troubleshooting Tips

If you can’t log into your mail server from a desktop mail client, scan your mail server to find if the ports are open. Note that you should run the following command from another Linux computer or server. If you run it on your mail server, then the ports will always appear to be open.

sudo nmap mail.your-domain.com

And check if Dovecot is running.

systemctl status dovecot

You can also check the mail log (/var/log/maillog), which may give you some clues. If Dovecot fails to start, the error might not be logged to the /var/log/maillog file, you can run the following command to see what’s wrong.

sudo journalctl -eu dovecot

For example, some folks may have the following error in the journal.

doveconf: Fatal: Error in configuration file /etc/dovecot/conf.d/10-master.conf line 78: Unknown setting

Most of the time, it’s a simple syntax error, like a missing curly bracket. Open the configuration file, go to the specified line and fix the error.

As I said in part 1, if you use Cloudflare DNS service, you should not enable the CDN (proxy) feature when creating DNS A record and AAAA record for the hostname of your mail server. Cloudflare doesn’t support SMTP or IMAP proxy.

If you use the iOS Mail app to log into your mail server and encounter the following error.

You can try to fix it by enforcing SSL encryption, for both SMTP and IMAP.

Fun fact: It seems the iOS Mail app has difficulty in supporting STARTTLS on IMAP port 143, but it supports STARTTLS on the submission port 587.

Auto-Renew TLS Certificate

You can create Cron job to automatically renew TLS certificate. Simply open root user’s crontab file.

sudo crontab -e

If you use Apache web server, add the following line at the bottom of the file.

@daily certbot renew --quiet && systemctl reload postfix dovecot httpd

If you are using Nginx web server, then add the following line.

@daily certbot renew --quiet && systemctl reload postfix dovecot nginx

Reloading Postfix, Dovecot and the web server is necessary to make these programs pick up the new certificate and private key.

Dovecot Automatic Restart

If for any reason your Dovecot process is killed, you need to run the following command to restart it.

sudo systemctl restart dovecot

Instead of manually typing this command, we can make Dovecot automatically restart by editing the dovecot.service systemd service unit. To override the default systemd service configuration, we create a separate directory.

sudo mkdir -p /etc/systemd/system/dovecot.service.d/

Then create a file under this directory.

sudo nano /etc/systemd/system/dovecot.service.d/restart.conf

Add the following lines in the file, which will make Dovecot automatically restart 5 seconds after a failure is detected.

[Service]
Restart=always
RestartSec=5s

Save and close the file. Then reload systemd.

sudo systemctl daemon-reload

To check if this would work, kill Dovecot with:

sudo pkill dovecot

Then check Dovecot status. You will find Dovecot automatically restarted.

systemctl status dovecot

Next Step

I hope this article helped you set up Postfix and Dovecot on CentOS 8/RHEL 8 server. In part 3, I will show you how to create virtual mailboxes on CentOS 8/RHEL 8 with PostfixAdmin.