Host Multiple Mail Domains in PostfixAdmin on CentOS/RHEL

This tutorial will be showing you how to set up multiple mail domains (virtual hosting) on CentOS/RHEL server with PostfixAdmin, which is an open-source web-based interface to configure and manage a Postfix based email server for many domains and users.

Prerequisites

To follow this tutorial, it’s required that

• You have configured PostfixAdmin with one mail domain.
• You have followed part 4 to set up SPF and DKIM.
• You have installed Roundcube Webmail.

Once the above requirements are met, follow the instructions below.

What You Need to Do

If you want to host multiple mail domains, then you need to

• Add a new mail domain and user in PostfixAdmin web-based panel.
• Create MX, A and SPF record for the new mail domain.
• Set up DKIM signing for the new domain.
• Create DMARC Record for the new domain.
• Set up RoundCube Webmail, Postfix and Dovecot for multiple domains

Step 1: Adding Additional Domains in PostfixAdmin Panel

Log into PostfixAdmin panel with the postmaster account. (https://postfixadmin.your-domain.com/) Then go to Domain List -> New Domain to add a new domain.

Note that the new domain should have DNS A record, or PostfixAdmin would throw the following error message. You can check the DNS propagation status on dnsmap.io.

Invalid domain domain2.com, and/or not discoverable in DNS

Next, add a user under the new domain.

Step 2: Creating MX, A and SPF record for the new mail domain

In your DNS manager, add MX record for the new domain like below.

Record Type    Name      Value

MX             @         mail.domain2.com

The A record points to your mail server’s IP address.

Record Type    Name     Value

A              mail     IP-address-of-mail-server

If your server uses IPv6 address, be sure to add AAAA record.

Then create SPF record to allow the MX host to send email for the new mail domain.

Record Type    Name      Value

TXT            @         v=spf1 mx ~all

Step 3: Setting up DKIM signing for the new domain

We have installed and configured OpenDKIM for a single domain in part 4 of this tutorial series. Now we need to tell OpenDKIM to sign every outgoing email for the new mail domain.

Edit the OpenDKIM signing table file.

sudo nano /etc/opendkim/SigningTable

Add the second domain like below.

*@domain1.com       20200308._domainkey.domain1.com
*@domain2.com       20200308._domainkey.domain2.com

Edit the key table file.

sudo nano /etc/opendkim/KeyTable

Add the second domain like below.

20200308._domainkey.domain1.com     domain1.com:20200308:/etc/opendkim/keys/domain1.com/20200308.private
20200308._domainkey.domain2.com     domain2.com:20200308:/etc/opendkim/keys/domain2.com/20200308.private

Edit the trusted hosts file.

sudo nano /etc/opendkim/TrustedHosts

Add the second domain like below.

127.0.0.1
localhost

.domain1.com
.domain2.com

Next, we need to generate a priavte/public keypair for the second domain. Create a separate folder for the second domain.

sudo mkdir /etc/opendkim/keys/domain2.com

Generate keys using opendkim-genkey tool.

sudo opendkim-genkey -b 2048 -d domain2.com -D /etc/opendkim/keys/domain2.com -s 20200308 -v

The above command will create 2048 bits keys. -d (domain) specifies the domain. -D (directory) specifies the directory where the keys will be stored and we use 20200308 (current date) as the selector (-s). Once the command is executed, the private key will be written to 20200308.private file and the public key will be written to 20200308.txt file.

Make opendkim as the owner of the private key.

sudo chown opendkim:opendkim /etc/opendkim/keys/domain2.com/20200308.private

Display the public key

sudo cat /etc/opendkim/keys/domain2.com/20200308.txt

The string after the p parameter is the public key.

In your DNS manager, create a TXT record for the second domain. Enter 20200308._domainkey in the Name field. Copy everything in the parentheses and paste into the value field. Delete all double-quotes. (You can paste it into a text editor first, delete all double quotes, then copy it to your DNS manager. Your DNS manager may require you to delete other invalid characters, such as carriage return.)

After saving your changes. Check the TXT record with this command.

dig TXT 20203008._domainkey.domain2.com

Now you can run the following command to test if your DKIM DNS record is correct.

sudo opendkim-testkey -d domain2.com -s 20200308 -vvv

If everything is OK, you will see

opendkim-testkey: using default configfile /etc/opendkim.conf
opendkim-testkey: checking key '20203008._domainkey.domain2.com'
opendkim-testkey: key secure
opendkim-testkey: key OK

If you see “Key not secure”, don’t panic. This is because DNSSEC isn’t enabled on your domain name. DNSSEC is a security standard for secure DNS query. Most domain names haven’t enabled DNSSEC. You can continue to follow this guide.

Restart OpenDKIM so it will start signing emails for the second domain.

sudo systemctl restart opendkim

Step 4: Creating DMARC Record For the New Domain

To create a DMARC record, go to your DNS manager and add a TXT record. In the name field, enter _dmarc. In the value field, enter the following. Note that you need to create the [email protected] email address.

v=DMARC1; p=none; pct=100; rua=mailto:[email protected]

The above DMARC record is a safe starting point. If you want to read a detailed explanation of DMARC, please check the following article.

• Creating DMARC Record to Protect Your Domain Name From Email Spoofing

Step 5: Setting up RoundCube, Postfix and Dovecot for Multiple Domains

I assume you installed Roundcube Webmail for the first mail domain.

It makes sense to let users of the first domain use mail.domain1.com and users of the second domain use mail.domain2.com when using RoundCube webmail. I will show you how to do it with Apache and Nginx.

Apache

If Roundcube is served by Apache web server, then create a virtual host for the second domain.

sudo nano /etc/httpd/conf.d/mail.domain2.com.conf

Put the following text into the file.

<VirtualHost *:80>
  ServerName mail.domain2.com
  DocumentRoot /var/www/roundcube/

  ErrorLog /var/log/httpd/mail.domain2.com_error.log
  CustomLog /var/log/httpd/mail.domain2.com_access.log combined

  <Directory />
    Options FollowSymLinks
    AllowOverride All
  </Directory>

  <Directory /var/www/roundcube/>
    Options FollowSymLinks MultiViews
    AllowOverride All
    Order allow,deny
    allow from all
  </Directory>

</VirtualHost>

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

sudo systemctl reload httpd

Nginx

If Roundcube is served by Nginx web server, then create a virtual host for the second domain.

sudo nano /etc/nginx/conf.d/mail.domain2.com.conf

Put the following text into the file.

server {
  listen 80;
  server_name mail.domain2.com;
  root /var/www/roundcube/;
  index index.php index.html index.htm;

  error_log /var/log/nginx/roundcube.error;
  access_log /var/log/nginx/roundcube.access;

  location / {
    try_files $uri $uri/ /index.php;
  }

  location ~ \.php$ {
   try_files $uri =404;
    fastcgi_pass unix:/run/php-fpm/www.sock;
    fastcgi_index index.php;
    fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
    include fastcgi_params;
  }

  location ~ /.well-known/acme-challenge {
    allow all;
  }
 location ~ ^/(README|INSTALL|LICENSE|CHANGELOG|UPGRADING)$ {
    deny all;
  }
  location ~ ^/(bin|SQL)/ {
    deny all;
  }
 # A long browser cache lifetime can speed up repeat visits to your page
  location ~* \.(jpg|jpeg|gif|png|webp|svg|woff|woff2|ttf|css|js|ico|xml)$ {
       access_log        off;
       log_not_found     off;
       expires           360d;
  }
}

Save and close the file. Then test Nginx configurations.

sudo nginx -t

If the test is successful, reload Nginx for the changes to take effect.

sudo systemctl reload nginx

Obtaining TLS Certificate

Now use Certbot to obtain TLS certificate for all your mail domains, so you will have a single TLS certificate with multiple domain names on it and mail clients won’t throw security warnings.

Apache

sudo certbot --apache --agree-tos --redirect --hsts --staple-ocsp -d mail.domain1.com,mail.domain2.com --cert-name mail.domain1.com --email [email protected]

Nginx

sudo certbot --nginx --agree-tos --redirect --hsts --staple-ocsp -d mail.domain1.com,mail.domain2.com --cert-name mail.domain1.com --email [email protected]

Notice that in the above command, we specified the cert name using the first mail domain, which will be used in the file path, so you don’t have to change the file path in Postfix or Dovecot configuration file.

When it asks if you want to update the existing certificate to include the new domain, answer U and hit Enter.

Now you should see the following message, which indicates the multi-domain certificate is successfully obtained.

Run the following command to grant permissions to read Let’s Encrypt TLS certificates.

Apache user

sudo setfacl -R -m u:apache:rx /etc/letsencrypt/live/ /etc/letsencrypt/archive/

Nginx user

sudo setfacl -R -m u:nginx:rx /etc/letsencrypt/live/ /etc/letsencrypt/archive/

Reload Apache or Nginx to pick up the new certificate.

sudo systemctl reload httpd
sudo systemctl reload nginx

You should now be able to use different domains to access RoundCube webmail. Also, you need to reload Postfix SMTP server and Dovecot IMAP server in order to let them pick up the new certificate. That’s all you need to do for Postfix and Dovecot to serve multiple domains.

sudo systemctl reload postfix dovecot

Using Mail Client on Your Computer or Mobile Device

Fire up your desktop email client such as Mozilla Thunderbird and add a mail account of the second domain.

• In the incoming server section, select IMAP protocol, enter mail.domain2.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.domain2.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.

Although Postfix SMTP server and Dovecot IMAP server are using the hostname of the first mail domain (mail.domain1.com) when communicating with others, they are now using a multi-domain certificate, so the mail client won’t display certificate warnings.

SPF and DKIM Check

Now you can use your desktop email client or webmail client to send a test email to [email protected] and get a free email authentication report. Here’s the report I got from port25.com

Don’t forget to test your email score at https://www.mail-tester.com and also test email placement with GlockApps.

If DKIM check fails, you can go to https://www.dmarcanalyzer.com/dkim/dkim-check/ to see if there are any errors with your DKIM record.

What if Your Emails Are Still Being Marked as Spam?

I have more tips for you in this article: 7 effective tips to stop your emails being marked as spam. Although it requires some time and effort, your emails will eventually be placed in the inbox after applying these tips.

rDNS for Multiple Mail Domains?

Reverse DNS (rDNS), aka PTR record, is used to check if the sender’s IP address matches the HELO hostname. You don’t need to add another PTR record when adding a new mail domain. Your rDNS should be set to just one hostname, i.e. the hostname set in Postfix, which can be shown with the following command.

postconf myhostname

rDNS is a record for IP addresses. It’s not a record for domain name. If you have just one IP address, then you need just one rDNS record pointing to one hostname. Reverse DNS check doesn’t verify what From: domain is in your email, as long as your IP address has an rDNS record and the hostname can resolve back to the same IP address, you can pass reverse DNS check.

Also, you don’t need to change the following settings in Postfix when hosting multiple domains on one server.

• myhostname
• myorigin
• mydestination

Hosted email services like G Suite and Microsoft Exchange Online all use one hostname for a server to send an email for their customers. Emails sent from G Suite users all use google.com in the hostname and emails sent from Microsoft Exchange Online users all use outlook.com in the hostname. This is also true for other email service providers like Mailchimp, SendinBlue, SendPluse, etc. They all use one hostname for a single server and one IP address has just one rDNS record.

Multiple TLS Certificates

If you manage email servers for lots of clients, you might not want to bundle all TLS certificates into one file, as it tells the information of your clients. Instead of using the following command to obtain a multi-domain certificate,

sudo certbot --nginx --agree-tos --redirect --hsts --staple-ocsp -d mail.domain1.com,mail.domain2.com

You can just use the following command to obtain a separate TLS certificate for the second mail domain.

sudo certbot --nginx --agree-tos --redirect --hsts --staple-ocsp -d mail.domain2.com

Next, you need to configure Postfix and Dovecot.

Postfix Multiple TLS Certificates

If you use Postfix version 3.4 or higher, you can use multiple TLS certificates. Edit the main.cf file.

sudo nano /etc/postfix/main.cf

Add the following line at the end of this file to enable multiple TLS certificates.

tls_server_sni_maps = hash:/etc/postfix/sni_maps

Save and close the file. Then create the sni_maps file.

sudo nano /etc/postfix/sni_maps

In this file, add each mail hostname and its certificate file.

mail.domain1.com   /etc/letsencrypt/live/mail.domain1.com/privkey.pem    /etc/letsencrypt/live/mail.domain1.com/fullchain.pem
mail.domain2.com   /etc/letsencrypt/live/mail.domain2.com/privkey.pem     /etc/letsencrypt/live/mail.domain2.com/fullchain.pem

Save and close the file. Next, build the lookup table.

sudo postmap -F /etc/postfix/sni_maps

Restart Postfix.

sudo systemctl restart postfix

You must use the -F option in postmap to base64-decode each value in the SNI map, or Postfix will throw the malformed BASE64 value error in the /var/log/mail.log file. The following command is wrong.

sudo postmap /etc/postfix/sni_maps

Dovecot Multiple TLS Certificates

If you use multiple TLS certificates in Postfix, you should also enable it in Dovecot. Edit the Dovecot 10-ssl.conf file.

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

You can find the following lines.

ssl_cert =</etc/letsencrypt/live/mail.domain1.com/fullchain.pem 
ssl_key =</etc/letsencrypt/live/mail.domain1.com/privkey.pem

Then add the following lines.

local_name mail.domain1.com {
ssl_cert =</etc/letsencrypt/live/mail.domain1.com/fullchain.pem
ssl_key =</etc/letsencrypt/live/mail.domain1.com/privkey.pem
}

local_name mail.domain2.com {
ssl_cert =</etc/letsencrypt/live/mail.domain2.com/fullchain.pem
ssl_key =</etc/letsencrypt/live/mail.domain2.com/privkey.pem
}

Save and close the file. Then restart Dovecot.

sudo systemctl restart dovecot

Cron Job

If you use SNI map in Postfix, then you should run the sudo postmap -F /etc/postfix/sni_maps command after TLS certificate is renewed. If you don’t do this, then email clients might complain your TLS certificate has expired. You can add this command to Crontab file, so it will be automated.

sudo crontab -e

Apache web server user:

@daily certbot renew --quiet; /usr/sbin/postmap -F /etc/postfix/sni_maps; systemctl reload postfix dovecot httpd

Wrapping Up

That’s it! I hope this tutorial helped you host multiple email domains with PostfixAdmin. As always, if you found this post useful, then subscribe to our free newsletter to get more tips and tricks. Take care 🙂