Please read the previous part first
This article is the seventh in the "Building a Mail Server on Linux" series.
In the previous part, we covered how to integrate Dovecot with PostgreSQL to set up user authentication.
If you haven't checked it yet, I recommend reading it first.
👉 Previous Part: Dovecot's auth-sql.conf.ext Configuration and Password Hashing

Postfix Configuration File Structure

Postfix is an SMTP server responsible for mail delivery, using various configuration files to adjust its operation.
This article explains the configuration file structure and key roles of Postfix.

Main Postfix Configuration Files

Main Postfix Configuration Items (main.cf)

Let’s take a look at the key configurations that define how Postfix operates. The file is located at /etc/postfix/main.cf.

1️⃣ Basic Mail Server Configuration

myhostname = mail.example.com
mydomain = example.com
myorigin = $mydomain
inet_interfaces = all
inet_protocols = ipv4

• myhostname → Hostname of the mail server
• mydomain → Default domain configuration
• inet_interfaces → Set to receive mail on all network interfaces
• inet_protocols → Use only IPv4 (can disable IPv6)

2️⃣ Mail Relay Restrictions

smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination

• permit_mynetworks → Allow relay from trusted networks
• permit_sasl_authenticated → Only SASL authenticated users can relay mail
• defer_unauth_destination → Block unauthenticated external relay

3️⃣ Virtual Users and Database Integration

To manage virtual domains and mailboxes using PostgreSQL, you need to add the following settings.

virtual_mailbox_domains = pgsql:/etc/postfix/sql/virtual_domains.cf
virtual_mailbox_maps = pgsql:/etc/postfix/sql/virtual_mailboxes.cf
virtual_alias_maps = pgsql:/etc/postfix/sql/virtual_aliases.cf

🔹 Explanation
- virtual_mailbox_domains → Queries the database for the virtual domains that can receive mail
- virtual_mailbox_maps → Queries the database for user mailbox information
- virtual_alias_maps → Queries the database for email aliases

💡 If using MySQL
You can use mysql: instead of pgsql:.

💡 In high-traffic environments, using proxy:pgsql: may provide performance benefits. However, it works sufficiently without proxy: in typical environments.

Setting Up Postfix to Use the Database

Postfix must have related packages installed to use PostgreSQL.

sudo apt update
sudo apt install postfix postfix-pgsql

• postfix → Basic Postfix SMTP server
• postfix-pgsql → Package allowing Postfix to integrate with PostgreSQL

Once the installation is complete, you will need to create configuration files that allow Postfix to fetch user information from PostgreSQL.

Postfix Database Integration Configuration (virtual_*.cf File Creation)

Postfix uses virtual_domains.cf, virtual_mailboxes.cf, and virtual_aliases.cf to retrieve information from the database.
These files need to be created manually, and you can create them based on the content below.

1️⃣ /etc/postfix/sql/virtual_domains.cf (Domain Query Configuration)

user = mailadmin
password = yourpassword
dbname = mail
hosts = 127.0.0.1
query = SELECT domain_name FROM mail_domain WHERE domain_name='%s' AND active=true

Explanation

user → PostgreSQL user account (make sure to create a user in the DB so that postfix can access the DB.) password → PostgreSQL account password (password for the previously created DB user mailadmin) dbname → Database name hosts → Database server address (using local server) query → Query to lookup the domains that can receive mail in the mail_domain table

2️⃣ /etc/postfix/sql/virtual_mailboxes.cf (Mailbox Query Configuration)

user = mailadmin
password = yourpassword
dbname = mail
hosts = 127.0.0.1
query = SELECT home_directory FROM mail_users WHERE email='%s'

Explanation

query → Queries the user's mail storage path (returns the home_directory field from the mail_users table) Postfix will return the path to save the user's mail in the correct directory

3️⃣ /etc/postfix/sql/virtual_aliases.cf (Email Alias Query Configuration)

user = mailadmin
password = yourpassword
dbname = mail
hosts = 127.0.0.1
query = SELECT destination_email FROM mail_alias WHERE source_email='%s'

Explanation

query → If there are aliases, queries the actual destination email where mail will be delivered Finds and returns the email registered as an alias by referring to the mail_alias table

Security Settings

These configuration files must have restricted permissions due to the inclusion of passwords.

sudo chmod 640 /etc/postfix/sql/virtual_*.cf
sudo chown root:postfix /etc/postfix/sql/virtual_*.cf

🔒 Security Note:

Since the configuration files contain passwords, permissions must be restricted so that other users cannot read them. By applying chmod 640, access is only allowed for the Postfix process (the postfix group).

Reloading Postfix Configuration

Once the configuration is complete, you need to reload Postfix to apply the changes.

sudo systemctl restart postfix

Summary and Next Steps

✅ Explanation of Postfix configuration file structure and roles
✅ Added key configurations in Postfix's main.cf
✅ Configured virtual_*.cf files for Postfix to integrate with PostgreSQL

In the next part, we will cover how to integrate Postfix with Dovecot for SMTP authentication.
This will ensure that the mail server is fully equipped for sending and receiving functionalities.

👉 Next Part: Integration of Postfix and Dovecot for SMTP Authentication (Coming Soon!)