Setting up a secure mail server

Reviewed on 27 March 2024 • Published on 04 June 2020

security
mailserver
DKIM
Rspamd
MariaDB
Roundcube
dmarc

In this tutorial you will learn how to configure a mail server that uses DKIM, Rspamd and MariaDB to deliver mails securely. You will install a Roundcube webmail interface to be able to read your mails directly from your browser.

Tip

We recommend you follow this tutorial using a Production-Optimized Instance.

Before you start

To complete the actions presented below, you must have:

A Scaleway account logged into the console
Owner status or IAM permissions allowing you to perform actions in the intended Organization
An SSH key
An Instance running Ubuntu Bionic Beaver or later
A domain or subdomain configured to point to the IP address of your Instance
Enabled the SMTP ports to send emails from your Instance

Pre-work

Important

Before you continue with this tutorial, some configuration is required to make sure your mail server will be working.

To ensure that other servers will accept mails sent from your Instance a valid reverse DNS within your own domain name (for example mail.domain.com) must be configured.
The SMTP ports have been unlocked in the security group of the server.

Start by updating your system to make sure you have the latest software releases installed:

apt update && apt upgrade

Before starting the installation of the mail server, make sure that there is no other mail software already installed:

service sendmail stop; update-rc.d -f sendmail remove

Tip

If you receive a message Failed to stop sendmail.service: Unit sendmail.service not loaded. you can ignore it. It will only tell you that sendmail has not been installed, so it cannot be removed.

Installing PostfixAdmin

All mailboxes will belong to virtual users. To manage mailboxes, we need one system user which will be the owner of all mailboxes and will be used by all virtual users to access their emails on the server. The home directory of the user will be /var/mail/vmail and all mailboxes will be stored in that directory:

groupadd -g 5000 vmail
useradd -u 5000 -g vmail -s /usr/sbin/nologin -d /var/mail/vmail -m vmail

As PostfixAdmin is a PHP application, a web server is required. We will use Nginx with PHP7.2 and MariaDB:

apt install nginx mariadb-server php-fpm php-cli php-imap php-json php-mysql php-opcache php-mbstring php-readline

Set a root password for MariaDB:

mysql_secure_installation

The setup tool will ask you the following questions:

Enter current password for root (enter for none): - Press Enter
Set root password? [Y/n] - Type Y
New password: - Enter the password for the root user
Re-enter new password: - Repeat the password
Remove anonymous users? [Y/n] - Type Y
Disallow root login remotely? [Y/n] - Type Y
Remove test database and access to it? [Y/n] - Type Y
Reload privilege tables now? [Y/n] - Type Y

Download and unpack PostfixAdmin. The latest version at the time of writing of this tutorial is version 3.3.8:

wget https://downloads.sourceforge.net/project/postfixadmin/postfixadmin-3.3.8/PostfixAdmin%203.3.8.tar.gz
tar xzf postfixadmin*.tar.gz

Move PostfixAdmin into the directory /var/www/postfixadmin:

mv postfixadmin-*/ /var/www/postfixadmin
rm -f PostfixAdmin*.tar.gz
mkdir /var/www/postfixadmin/templates_c

Change the ownership of the directory to the www-data user, as Nginx and PHP are using it:

chown -R www-data: /var/www/postfixadmin

PostfixAdmin will use a MySQL database to store information. Connect to your MariaDB Server to create a new database and user:

mysql -u root -p

Create the database, remember to replace your_secret_password with a password for the postfixadmin user:

CREATE DATABASE postfixadmin;
GRANT ALL ON postfixadmin.* TO 'postfixadmin'@'localhost' IDENTIFIED BY 'your_secret_password';
FLUSH PRIVILEGES;
EXIT;

Create a configuration file /var/www/postfixadmin/config.local.php and open it in a text editor. Copy the following content into it:

<?php
$CONF['configured'] = true;
$CONF['database_type'] = 'mysqli';
$CONF['database_host'] = 'localhost';
$CONF['database_user'] = 'postfixadmin';
$CONF['database_password'] = 'your_secret_password';
$CONF['database_name'] = 'postfixadmin';
$CONF['default_aliases'] = array (
'abuse'      => 'abuse@example.com',
'hostmaster' => 'hostmaster@example.com',
'postmaster' => 'postmaster@example.com',
'webmaster'  => 'webmaster@example.com'
);
$CONF['fetchmail'] = 'NO';
$CONF['show_footer_text'] = 'NO';
$CONF['quota'] = 'YES';
$CONF['domain_quota'] = 'YES';
$CONF['quota_multiplier'] = '1024000';
$CONF['used_quotas'] = 'YES';
$CONF['new_quota_table'] = 'YES';
$CONF['aliases'] = '0';
$CONF['mailboxes'] = '0';
$CONF['maxquota'] = '0';
$CONF['domain_quota_default'] = '0';
?>

Note

Remember to replace your_secret_password with the password for the database user.

The configuration defines the database type, login credentials, default aliases, disabled fetchmail and enabled quota.

Now run the following script to install the database schema:

sudo -u www-data php /var/www/postfixadmin/public/upgrade.php

As the database is ready now, it is possible to create the first superadmin from the CLI tools:

bash /var/www/postfixadmin/scripts/postfixadmin-cli admin add

Enter the email address of the admin, and answer the questions of the CLI.

Create a Nginx configuration file for your domain. Open the file /etc/nginx/sites-enabled/mail.example.com.conf and paste the following configuration. Do not forget to edit the server_name to your domain name:

server {
    listen 80;
    server_name mail.example.com;
    root /var/www;
    location / {
    try_files $uri $uri/ /index.php;
    }
    location /postfixadmin {
    index index.php;
    try_files $uri $uri/ /postfixadmin/public/login.php;
    }
    location ~* \.php$ {
        fastcgi_split_path_info ^(.+?\.php)(/.*)$;
        if (!-f $document_root$fastcgi_script_name) {return 404;}
        fastcgi_pass  unix:/run/php/php7.4-fpm.sock;
        fastcgi_index index.php;
        include fastcgi_params;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
    }
}

Restart Nginx to activate the configuration:

service nginx restart

To secure the communication with the web server, we use Let’s Encrypt to get a free SSL certificate:

apt install python3-certbot-nginx
certbot --nginx

Installation of Postfix and Dovecot

Install the required software. You can install all required packages with one command with apt:
```
apt install postfix postfix-mysql dovecot-imapd dovecot-lmtpd dovecot-pop3d dovecot-mysql
```
You will be asked some questions during the installation:
- For server type choose Internet Site
- For mail name, enter the FQDN of the server (for example: mail.example.com)
We use virtual users in our configuration. Therefore, we have to create the configuration files for postfix to use the database we have created previously.

Start by creating a directory to store the files:

mkdir -p /etc/postfix/sql

Create and open the file /etc/postfix/sql/mysql_virtual_domains_maps.cf in a text editor and put the following content in it:

user = postfixadmin
password = your_secret_password
hosts = 127.0.0.1
dbname = postfixadmin
query = SELECT domain FROM domain WHERE domain='%s' AND active = '1'

Create and open the file /etc/postfix/sql/mysql_virtual_alias_maps.cf in a text editor and put the following content in it:

user = postfixadmin
password = your_secret_password
hosts = 127.0.0.1
dbname = postfixadmin
query = SELECT goto FROM alias WHERE address='%s' AND active = '1'

Create and open the file /etc/postfix/sql/mysql_virtual_alias_domain_catchall_maps.cf in a text editor and put the following content in it:

user = postfixadmin
password = your_secret_password
hosts = 127.0.0.1
dbname = postfixadmin
query  = SELECT goto FROM alias,alias_domain WHERE alias_domain.alias_domain = '%d' and alias.address = CONCAT('@', alias_domain.target_domain) AND alias.active = 1 AND alias_domain.active='1'

Create and open the file /etc/postfix/sql/mysql_virtual_alias_domain_maps.cf in a text editor and put the following content in it:

user = postfixadmin
password = your_secret_password
hosts = 127.0.0.1
dbname = postfixadmin
query = SELECT goto FROM alias,alias_domain WHERE alias_domain.alias_domain = '%d' and alias.address = CONCAT('%u', '@', alias_domain.target_domain) AND alias.active = 1 AND alias_domain.active='1'

Create and open the file /etc/postfix/sql/mysql_virtual_mailbox_maps.cf in a text editor and put the following content in it:

user = postfixadmin
password = your_secret_password
hosts = 127.0.0.1
dbname = postfixadmin
query = SELECT maildir FROM mailbox WHERE username='%s' AND active = '1'

Create and open the file /etc/postfix/sql/mysql_virtual_alias_domain_mailbox_maps.cf in a text editor and put the following content in it:

user = postfixadmin
password = your_secret_password
hosts = 127.0.0.1
dbname = postfixadmin
query = SELECT maildir FROM mailbox,alias_domain WHERE alias_domain.alias_domain = '%d' and mailbox.username = CONCAT('%u', '@', alias_domain.target_domain) AND mailbox.active = 1 AND alias_domain.active='1'

Once the MySQL configuration files are created, update the configuration of Postfix:

postconf -e "virtual_mailbox_domains = mysql:/etc/postfix/sql/mysql_virtual_domains_maps.cf"
postconf -e "virtual_alias_maps = mysql:/etc/postfix/sql/mysql_virtual_alias_maps.cf, mysql:/etc/postfix/sql/mysql_virtual_alias_domain_maps.cf, mysql:/etc/postfix/sql/mysql_virtual_alias_domain_catchall_maps.cf"
postconf -e "virtual_mailbox_maps = mysql:/etc/postfix/sql/mysql_virtual_mailbox_maps.cf, mysql:/etc/postfix/sql/mysql_virtual_alias_domain_mailbox_maps.cf"

Note

The postconf command can be used to display the actual configuration, change configuration values, or display other configuration information about the Postfix mail system.

Local delivery will be handled by Dovecot’s local delivery agent. It will take mail from an MTA (Postfix) and deliver it to a local user’s mailbox.

postconf -e "virtual_transport = lmtp:unix:private/dovecot-lmtp"

Configure TLS parameters by using the Let’s encrypt SSL certificate:

postconf -e 'smtp_tls_security_level = may'
postconf -e 'smtpd_tls_security_level = may'
postconf -e 'smtp_tls_note_starttls_offer = yes'
postconf -e 'smtpd_tls_loglevel = 1'
postconf -e 'smtpd_tls_received_header = yes'
postconf -e 'smtpd_tls_cert_file = /etc/letsencrypt/live/mail.example.com/fullchain.pem'
postconf -e 'smtpd_tls_key_file = /etc/letsencrypt/live/mail.example.com/privkey.pem'

Configure the authenticated SMTP settings:

postconf -e 'smtpd_sasl_type = dovecot'
postconf -e 'smtpd_sasl_path = private/auth'
postconf -e 'smtpd_sasl_local_domain ='
postconf -e 'smtpd_sasl_security_options = noanonymous'
postconf -e 'broken_sasl_auth_clients = yes'
postconf -e 'smtpd_sasl_auth_enable = yes'
postconf -e 'smtpd_recipient_restrictions = permit_sasl_authenticated,permit_mynetworks,reject_unauth_destination'

Enable the TLS/SSL and submission ports in the Postfix configuration file. Open the file /etc/postfix/master.cf with a text editor, uncomment the submission and smtps sections as following. Make sure that there is a whitespace in front of the -o as it is required:

submission inet n       -       y       -       -       smtpd
-o syslog_name=postfix/submission
-o smtpd_tls_security_level=encrypt
-o smtpd_sasl_auth_enable=yes
#  -o smtpd_reject_unlisted_recipient=no
-o smtpd_client_restrictions=permit_sasl_authenticated,reject
#  -o smtpd_helo_restrictions=$mua_helo_restrictions
#  -o smtpd_sender_restrictions=$mua_sender_restrictions
#  -o smtpd_recipient_restrictions=
#  -o smtpd_relay_restrictions=permit_sasl_authenticated,reject
-o milter_macro_daemon_name=ORIGINATING
smtps     inet  n       -       y       -       -       smtpd
-o syslog_name=postfix/smtps
-o smtpd_tls_wrappermode=yes
-o smtpd_sasl_auth_enable=yes
#  -o smtpd_reject_unlisted_recipient=no
-o smtpd_client_restrictions=permit_sasl_authenticated,reject
#  -o smtpd_helo_restrictions=$mua_helo_restrictions
#  -o smtpd_sender_restrictions=$mua_sender_restrictions
#  -o smtpd_recipient_restrictions=
#  -o smtpd_relay_restrictions=permit_sasl_authenticated,reject
-o milter_macro_daemon_name=ORIGINATING

Restart postfix to take the modifications into effect:

service postfix restart

Edit the file /etc/dovecot/dovecot-sql.conf.ext as follows:

driver = mysql
connect = host=127.0.0.1 dbname=postfixadmin user=postfixadmin password=your_secret_password
default_pass_scheme = MD5-CRYPT
iterate_query = SELECT username AS user FROM mailbox
user_query = SELECT CONCAT('/var/mail/vmail/',maildir) AS home, \
CONCAT('maildir:/var/mail/vmail/',maildir) AS mail, \
5000 AS uid, 5000 AS gid, CONCAT('*:bytes=',quota) AS quota_rule \
FROM mailbox WHERE username = '%u' AND active = 1
password_query = SELECT username AS user,password FROM mailbox \
WHERE username = '%u' AND active='1'

Edit the file /etc/dovecot/conf.d/10-mail.conf with the required information as shown below:

...
mail_location = maildir:/var/mail/vmail/%d/%n
...
mail_uid = vmail
mail_gid = vmail
...
first_valid_uid = 5000
last_valid_uid = 5000
...
mail_privileged_group = mail
...
mail_plugins = quota
...

Modify the information in the file /etc/dovecot/conf.d/10-auth.conf as following:

...
disable_plaintext_auth = yes
...
auth_mechanisms = plain login
...
#!include auth-system.conf.ext
!include auth-sql.conf.ext
...

Edit the file /etc/dovecot/conf.d/10-master.conf as following:

...
service lmtp {
unix_listener /var/spool/postfix/private/dovecot-lmtp {
    mode = 0600
    user = postfix
    group = postfix
}
...
}
...
service auth {
...
unix_listener auth-userdb {
    mode = 0600
    user = vmail
    group = vmail
}
...
unix_listener /var/spool/postfix/private/auth {
    mode = 0666
    user = postfix
    group = postfix
}
...
}
...
service auth-worker {
user = vmail
}
...
service dict {
unix_listener dict {
    mode = 0660
    user = vmail
    group = vmail
}
}
...

Edit the file /etc/dovecot/conf.d/10-ssl.conf as following:

...
ssl = yes
...
ssl_cert = </etc/letsencrypt/live/mail.example.com/fullchain.pem
ssl_key = </etc/letsencrypt/live/mail.example.com/privkey.pem
ssl_dh = </etc/ssl/certs/dhparam.pem
...
ssl_cipher_list = EECDH+AES:EDH+AES+aRSA
...
ssl_prefer_server_ciphers = yes
...

Add the following line to the file /etc/dovecot/conf.d/20-imap.conf

...
protocol imap {
...
mail_plugins = $mail_plugins imap_quota
...
}
...

Edit the file /etc/dovecot/conf.d/20-lmtp.conf as following:

...
protocol imap {
postmaster_address = postmaster@example.com
mail_plugins = $mail_plugins
}
...

Add a spam folder in the file /etc/dovecot/conf.d/15-mailboxes.conf:

...
mailbox Drafts {
special_use = \Drafts
}
mailbox Spam {
special_use = \Junk
auto = subscribe
}
mailbox Junk {
special_use = \Junk
}
...

Configure Dovecot to connect to the MySQL database to manage the quotas, either per domain or per user. It will send a notification by email once the mailbox of a user has reached a certain level of saturation. Edit the file /etc/dovecot/conf.d/90-quota.conf:

quota = dict:User quota::proxy::sqlquota
quota_rule = *:storage=5GB
quota_rule2 = Trash:storage=+100M
quota_grace = 10%%
quota_exceeded_message = Quota exceeded, please contact your system administrator.
quota_warning = storage=100%% quota-warning 100 %u
quota_warning2 = storage=95%% quota-warning 95 %u
quota_warning3 = storage=90%% quota-warning 90 %u
quota_warning4 = storage=85%% quota-warning 85 %u
}
service quota-warning {
executable = script /usr/local/bin/quota-warning.sh
user = vmail
unix_listener quota-warning {
group = vmail
mode = 0660
user = vmail
}
}
dict {
sqlquota = mysql:/etc/dovecot/dovecot-dict-sql.conf.ext

Make sure your MySQL credentials are correct then edit the file /etc/dovecot/dovecot-dict-sql.conf.ext and add the following content:

...
connect = host=127.0.0.1 dbname=postfixadmin user=postfixadmin password=your_secret_password
...
map {
pattern = priv/quota/storage
table = quota2
username_field = username
value_field = bytes
}
map {
pattern = priv/quota/messages
table = quota2
username_field = username
value_field = messages
}
...
# map {
#   pattern = shared/expire/$user/$mailbox
#   table = expires
#   value_field = expire_stamp
#
#   fields {
#     username = $user
#     mailbox = $mailbox
#   }
# }
...

Create the following script, which sends a warning to a user that reaches the capacity limits of his mailbox. Edit the file /usr/local/bin/quota-warning.sh:

#!/bin/sh
PERCENT=$1
USER=$2
cat << EOF | /usr/lib/dovecot/dovecot-lda -d $USER -o "plugin/quota=dict:User quota::noenforcing:proxy::sqlquota"
From: postmaster@example.com
Subject: Quota warning
Your mailbox is $PERCENT% full. Do not forget to make a backup of old messages to remain able to receive mails.
EOF

Make the script executable:

chmod +x /usr/local/bin/quota-warning.sh

And restart dovecot:

service dovecot restart

Your mail server is now ready to be used, and you can create mailboxes at http://your_servers_ip/postfixadmin

Installing Rspamd

To protect yourself from spam and to increase the trust level of your mails, we use Rspamd and create DKIM and DMARC DNS records.

Installing Redis

Redis is used by Rspamd as a storage and caching system. It can easily be installed with apt:

apt install redis-server

Installing Rspamd

Add the repository of Rspamd to your apt sources with the following command:

wget -O- https://rspamd.com/apt-stable/gpg.key | apt-key add -
echo "deb http://rspamd.com/apt-stable/ $(lsb_release -cs) main" | tee -a /etc/apt/sources.list.d/rspamd.list

Then update your apt cache and install the tool:

apt update
apt install rspamd

Configuring Rspamd

Once the installation has finished, create the config files for Rspamd in the directory /etc/rspamd/local.d/local.d/.

The file /etc/rspamd/local.d/worker-normal.inc contains information about the port on which Rspamd listens. The default port is 11333, bind it to your localhost by adding the following line to the file:

bind_socket = "127.0.0.1:11333";

Configure a proxy between Postfix and Rspamd, it listens on port 11332 and uses milter to communicate between the two tools. Edit the file /etc/rspamd/local.d/worker-proxy.inc

Set a password for the worker. To generate an encrypted password, run the following command:

rspamadm pw --encrypt -p your_secret_password

Note

Remember to replace your_secret_password with your own password

You will see an output like the following:

rspamadm pw --encrypt -p your_secret_password
$2$93qin9nkifzjpr7taqhs9guua888tnny$dnys6um6xm1gb1amgnz9hocuz7grxuk5z9yjw87psrk6yu641oiy

Edit the file /etc/rspamd/local.d/worker-controller.inc and put the encrypted password into it:

password = "$2$93qin9nkifzjpr7taqhs9guua888tnny$dnys6um6xm1gb1amgnz9hocuz7grxuk5z9yjw87psrk6yu641oiy";

Configure redis to be used with rspamd by editing the file /etc/rspamd/local.d/classifier-bayes.conf:

servers = "127.0.0.1";
backend = "redis";

Set the milter headers in the file /etc/rspamd/local.d/milter_headers.conf:
```
use = ["x-spamd-bar", "x-spam-level", "authentication-results"];
```

Restart Rspamd:

service rspamd restart

Add a proxy in the Nginx configuration file (for example: /etc/nginx/sites-enabled/mail.example.com.conf):

...
location /rspamd {
    proxy_pass http://127.0.0.1:11334/;
    proxy_set_header Host $host;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
}
...

Restart Nginx:

service nginx restart

You can access the web interface of Rspamd now at: http://your_servers_ip/rspamd. To authenticate use the password you have set with the rspamadm pw command.

Configuring Postfix

To use Rspamd with Postfix, the configuration has to be updated with postconf:

postconf -e "milter_protocol = 6"
postconf -e "milter_mail_macros = i {mail_addr} {client_addr} {client_name} {auth_authen}"
postconf -e "milter_default_action = accept"
postconf -e "smtpd_milters = inet:127.0.0.1:11332"
postconf -e "non_smtpd_milters = inet:127.0.0.1:11332"

Restart postfix to take the changes into effect:

service postfix restart

Configuring Dovecot

Dovecot is already installed on the server, add the sieve filtering module and integrate it with Rspamd.

Install sieve via apt:

apt install dovecot-sieve dovecot-managesieved

Open the file /etc/dovecot/conf.d/20-lmtp.conf and edit it as following:

...
protocol lmtp {
postmaster_address = postmaster@example.com
mail_plugins = $mail_plugins sieve
}
...

Open the file /etc/dovecot/conf.d/20-imap.conf and edit it as following:

...
protocol imap {
...
mail_plugins = $mail_plugins imap_quota imap_sieve
...
}
...

Edit the file /etc/dovecot/conf.d/20-managesieve.conf as following:

...
service managesieve-login {
inet_listener sieve {
    port = 4190
}
}
...
service managesieve {
process_limit = 1024
}
...

Open and edit the file /etc/dovecot/conf.d/90-sieve.conf as following:

plugin {
    ...
    # sieve = file:~/sieve;active=~/.dovecot.sieve
    sieve_plugins = sieve_imapsieve sieve_extprograms
    sieve_before = /var/vmail/mail/sieve/global/spam-global.sieve
    sieve = file:/var/vmail/mail/sieve/%d/%n/scripts;active=/var/vmail/mail/sieve/%d/%n/active-script.sieve
    imapsieve_mailbox1_name = Spam
    imapsieve_mailbox1_causes = COPY
    imapsieve_mailbox1_before = file:/var/vmail/mail/sieve/global/report-spam.sieve
    imapsieve_mailbox2_name = *
    imapsieve_mailbox2_from = Spam
    imapsieve_mailbox2_causes = COPY
    imapsieve_mailbox2_before = file:/var/vmail/mail/sieve/global/report-ham.sieve
    sieve_pipe_bin_dir = /usr/bin
    sieve_global_extensions = +vnd.dovecot.pipe
    ....
}

Restart the dovecot service and create a directory for the sieve scripts:

service dovecot restart
mkdir -p /var/vmail/mail/sieve/global

Create a global sieve filter in the file /var/vmail/mail/sieve/global/spam-global.sieve. It will move emails marked as spam directly to the spam folder:

require ["fileinto","mailbox"];
if anyof(
    header :contains ["X-Spam-Flag"] "YES",
    header :contains ["X-Spam"] "Yes",
    header :contains ["Subject"] "*** SPAM ***"
    )
{
    fileinto :create "Spam";
    stop;
}

Create a script, named /var/vmail/mail/sieve/global/report-spam.sieve, that will be triggered each time you manually move an email into the spam folder:

require ["vnd.dovecot.pipe", "copy", "imapsieve"];
pipe :copy "rspamc" ["learn_spam"];

Create a script, named /var/vmail/mail/sieve/global/report-ham.sieve, that will be triggered each time when you move an email out of the spam folder:

require ["vnd.dovecot.pipe", "copy", "imapsieve"];
pipe :copy "rspamc" ["learn_ham"];

Compile the sieve scripts and set permissions:

sievec /var/vmail/mail/sieve/global/spam-global.sieve
sievec /var/vmail/mail/sieve/global/report-spam.sieve
sievec /var/vmail/mail/sieve/global/report-ham.sieve
chown -R vmail: /var/vmail/mail/sieve/

Creating DKIM keys

DomainKeys Identified Mail (DKIM) is an email authentication method designed to detect email spoofing. It allows the receiving server to verify the origin of an email by affixing a digital signature to it. Verification of the signature is carried out using the signer’s public key published in the DNS. It can be used to detect fraudulent emails.

Create a new directory to store the DKIM key and generate a new DKIM key pair by using the rspamadm utility. In the following example we use mail as a DKIM selector. It will generate a key pair that can be used for all domains handled by the mail server.

mkdir /var/lib/rspamd/dkim/
rspamadm dkim_keygen -b 2048 -s mail -k /var/lib/rspamd/dkim/mail.key > /var/lib/rspamd/dkim/mail.pub

You will find two files in the directory:

mail.key - The privatse key file
mail.pub - The public key file

Create a new file /etc/rspamd/local.d/dkim_signing.conf to tell Rspamd where to look for the DKIM key and the selector name. The last line will enable DKIM signing for alias sender addresses:

selector = "mail";
path = "/var/lib/rspamd/dkim/$selector.key";
allow_username_mismatch = true;

Rspamd supports also ARC signatures, use the same configuration file and copy it

cp  /etc/rspamd/local.d/dkim_signing.conf /etc/rspamd/local.d/arc.conf

Restart Rspamd:

service rspamd restart

DNS settings

To sign your mails with DKIM, you have to add the public key to your DNS zone.

Retrieve the key:

cat /var/lib/rspamd/dkim/mail.pub

An output like the following one will display:

mail._domainkey IN TXT ( "v=DKIM1; k=rsa; "
    "p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAxzllrHkbUwSR24B6iG+TgNTOU43lZhTPJemo8PKXkgaVppGJ57tlWOe9U321Qk+Ksk9qoukR4f39TCMQdAgtEPFpWSBWJJE9C2SmNz38SmhTC0AkvIzBxCdatitK2aWjHq4s9bsoQ1gIQlXKM+V7GbN2LFCBfvAU7ElBQk+QG2DuxGD/XNDLQWloYEWcqkUfxlHr0znoY86jkglVZ"
    "nhi/cAoE0SbzjphCtibT9T1w6AztxV1yK2VTJPpBFdtAsP1Sa3GDbTn0HATHUJI8eOIXtFcIBbYisiOIWjisE3TXFpvkS69Q0gvxVFYDnftLvsf5AticeygdMOVbK1o3T4Z7QIDAQAB"
) ;

You can add the information as a TXT record in the DNS zone of your domain.

Installing Roundcube Webmail

To comfortably read your mails directly within your web browser, we will install a Roundcube web mail interface.

Start by installing all PHP dependencies:

apt install php-intl php-mail-mime php-net-smtp php-net-socket php-pear php-xml php-intl php-xml php-gd php-gd php-imagick

Log into your MariaDB server and create a MySQL database for Roundcube:

mysql -u root -p

CREATE DATABASE roundcubemail;
GRANT ALL ON roundcubemail.* TO 'roundcube'@'localhost' IDENTIFIED BY 'your_secret_password';
FLUSH PRIVILEGES;
\q

Download Roundcube, unpack it and move it into the web directory:

wget https://github.com/roundcube/roundcubemail/releases/download/1.6.1/roundcubemail-1.6.1-complete.tar.gz
tar xzf roundcubemail-1.6.-complete.tar.gz
mv roundcubemail-1.6.1 /var/www/webmail
rm roundcubemail-1.6.1-complete.tar.gz

Change the ownership of the folder and all files in it to the www-data user:

chown -R www-data: /var/www/webmail

Edit the Nginx configuration (/etc/nginx/sites-enabled/mail.example.com) of your server and add the information for the Roundcube directory:

...
location /webmail {
index index.php;
try_files $uri $uri/ /webmail/index.php;
}
location ~ ^/webmail/(README|INSTALL|LICENSE|CHANGELOG|UPGRADING)$ {
deny all;
}
location ~ ^/webmail/(bin|SQL|config|temp|logs)/ {
deny all;
}
...

Restart Nginx:

service nginx restart

Launch installation from https://your_server_ip/webmail/installer/ and enter all the required information.

Remove the installer for security reasons:

rm -rf /var/www/roundcubemail/installer