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

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
Reverse DNS check 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.

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.

postfixadmin multiple domains

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.

postfixadmin add new mailbox

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.)

DKIM records

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 dmarc@domain2.com email address.

v=DMARC1; p=none; pct=100; rua=mailto:dmarc@domain2.com

create dmarc record txt

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

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

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.

Apache

sudo /usr/local/bin/certbot --apache --agree-tos --redirect --hsts --staple-ocsp -d mail.domain1.com,mail.domain2.com --cert-name mail.domain1.com --email you@example.com

Nginx

sudo /usr/local/bin/certbot --nginx --agree-tos --redirect --hsts --staple-ocsp -d mail.domain1.com,mail.domain2.com --cert-name mail.domain1.com --email you@example.com

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.

certbot letsencrypt multi-domain postfixadmin

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

postfixadmin roundcube multiple domains

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.

postfixadmin-mail-server-desktop-mail-client-configuration

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 check-auth@verifier.port25.com and get a free email authentication report. Here’s the report I got from port25.com

postfix spf dkim ubuntu

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

What if Your Emails Are Still Being Marked as Spam?

I have more tips for you in this article: How 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.

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 🙂

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6 Responses to “Host Multiple Mail Domains in PostfixAdmin on CentOS/RHEL

  • Will the fact that I have one server hosting multiple domains cause an issue with dkim or spamassian or any other item. The PRT record for the IP points to the mail server and it’s domain. Will adding additional domains cause problems with mail delivery?

  • Hi,

    I’ve added my second domain.
    I have the error message “postfix/smtpd[12493]: NOQUEUE: reject: RCPT […] Relay access denied; […]” in my “/var/log/maillog” with other lines “warning: connect to Milter service inet:127.0.0.1:8893: Connection refused”

    I don’t know where is the problem. I can send email from my second domain with Roundcube (on this server), but i can’t send with thunderbird : REJECTED.

    Can you help me ? I don’t find the solution…

    Thanks for your wonderful documentation !

    • My bad, I configured Thunderbird with port 25 for SMTP. With the use of port 587, it’s working better.

  • Matthew Scott
    4 weeks ago

    How do I fix the issue with DKIM & SPF not passing for virtual domains, because they’re sent using the mail server domain?

    So, my mail server is: mail.domain1.com, and has been set up in line with everything from your tutorial.

    Virtual domain: domain2.com with email: user@domain2.com – email sent to gmail address states SPF as neutral, and then in my server logs it states that there no signing table match / no signature data for ‘user@domain2.com’ (because the data in the signing table is set to email addresses with “@domain1.com” in them).

    Is there a way for the steps from this tutorial to be automated when adding new domains with postfix admin? So, when adding domains to postfix admin the “Create Signing Table, Key Table and Trusted Hosts File”, “Generate Private/Public Keypair” & “Publish Your Public Key in DNS Records” steps are done automatically and the data for DNS TXT records is emailed to a specific email address so that who ever is in charge of setting up DNS related tasks can simply add the correct TXT record to the DNS records.

    Is this possible?

    Thanks – great tutorial by the way. I’ve learnt a lot more about what’s going on behind the scenes by following your tutorials!

    Matt

    • If you followed step 3 correctly, then emails sent from domain2.com will be DKIM signed.

      PostfixAdmin doesn’t provide an automatic way to add new domains for DKIM signing. Modoboa is better. However, Modoboa currently doesn’t support CentOS 8.

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