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
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.
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 Amavis for a single domain in part 4 of this tutorial series. Now we need to make Amavis sign every outgoing email for the new mail domain. Edit the Amavis configuration file.
sudo nano /etc/amavisd/amavisd.conf
Find the following line. Note that 20200119 is the DKIM selector for my domain name. You might have a different DKIM selector.
dkim_key('domain1.com', '20200119', '/var/spool/amavisd/dkim/domain1.com.pem');
Add another line to specify the DKIM selector and location of the private key for the second domain. You can use whatever name for the DKIM selector, but I found it’s convienent to use the current date (Feburary 24, 2020) as the DKIM selector.
dkim_key('domain2.com', '20200224', '/var/spool/amavisd/dkim/domain2.com.pem');
In @dkim_signature_options_bysender_maps section, you have
"domain1.com" => { d => "domain1.com", a => 'rsa-sha256', ttl => 10*24*3600 },
Now we need to add a new line for the second domain.
"domain2.com" => { d => "domain2.com", a => 'rsa-sha256', ttl => 10*24*3600 },
Like this:
@dkim_signature_options_bysender_maps = ( {
# 'd' defaults to a domain of an author/sender address,
# 's' defaults to whatever selector is offered by a matching key
# explicit 'd' forces a third-party signature on foreign (hosted) domains
"domain1.com" => { d => "domain1.com", a => 'rsa-sha256', ttl => 10*24*3600 },
"domain2.com" => { d => "domain2.com", a => 'rsa-sha256', ttl => 10*24*3600 },
# catchall defaults
'.' => { a => 'rsa-sha256', c => 'relaxed/simple', ttl => 30*24*3600 },
} );
Save and close the file. Then generate the private key for the second domain.
sudo amavisd genrsa /var/spool/amavisd/dkim/domain2.com.pem 2048
Restart Amavis.
sudo systemctl restart amavisd
Display the public keys.
sudo amavisd -c /etc/amavisd/amavisd.conf showkeys
All public keys will be displayed. We need the public key of the second domain, which is in the parentheses.
In you DNS manager, create a TXT record, enter 20200224._domainkey in the name field. If you used a different DKIM selector, replace 20200224 with your real DKIM selector.
Then go back to the terminal window, copy everything in the parentheses and paste it into the value field of the DNS record. You need to delete all double quotes and line breaks in the value field. If you don’t delete them, then key test will probably fail.
After saving your changes. Check the TXT record with this command.
dig TXT 20200224._domainkey.domain2.com
Now you can run the following command to test if your DKIM DNS record is correct.
sudo amavisd -c /etc/amavisd/amavisd.conf testkeys
If the DNS record is correct, the test will pass.
TESTING#1 domain1.com: 20200119._domainkey.domain1.com => pass TESTING#2 domain2.com: 20200224._domainkey.domain2.com => pass
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.
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.
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 Options FollowSymLinks AllowOverride All Options FollowSymLinks MultiViews AllowOverride All Order allow,deny allow from all
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 [email protected]
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 [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.
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.comas the server name, choose port 143 and STARTTLS. Choosenormal passwordas the authentication method. - In the outgoing section, select SMTP protocol, enter
mail.domain2.comas the server name, choose port 587 and STARTTLS. Choosenormal passwordas the authentication method.
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.
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 🙂
