By: U. S. Siemer

Published:

Postfix Setup for Aliases/Forwarding Across Multiple Domains

Managing email for multiple domains on a single server, especially when you only need to forward or alias incoming mail to existing users or external addresses, is a common requirement. Postfix, a powerful Mail Transfer Agent (MTA), provides a robust mechanism for this using what it terms "virtual alias domains" and "virtual alias maps."

While the term "virtual" might suggest complex setups like virtual machines or dedicated mailboxes, in this context, it simply refers to Postfix's ability to handle email for domains that are not directly your server's primary hostname. This guide will walk you through configuring Postfix specifically for this alias and forwarding functionality.

Goal of This Configuration

Our objective is to configure a single Postfix instance to accept email for multiple distinct domains (e.g., domain1.com, domain2.net, domain3.org) and then, based on the recipient's address, forward that email to either:

This setup does not involve creating individual mailboxes on your server for each of the virtual email addresses.

Prerequisites

Before you begin, ensure the following:

Step-by-Step Configuration

Step 1: Configure Postfix's main.cf

The main.cf file is the core configuration file for Postfix. You'll specify your server's identity and declare which domains Postfix will handle aliases for.

Open the file for editing:

sudo nano /etc/postfix/main.cf

Add or modify the following lines. Ensure that your actual domains are NOT listed in mydestination, as that would cause Postfix to attempt local delivery rather than using the alias maps.

    # Basic server identity:
    # myhostname: The fully qualified domain name of your mail server.
    #             This is often a subdomain, e.g., mail.yourserver.com.
    myhostname = mail.your_server_fqdn.com

    # mydomain: Your primary domain, derived from myhostname.
    mydomain = your_primary_domain.com

    # mydestination: Domains for which this server is the final delivery destination
    #                for local UNIX users. Crucially, DO NOT list your virtual alias domains here.
    mydestination = $myhostname, localhost.$mydomain, localhost

    # virtual_alias_domains: Declare all domains for which Postfix will accept mail
    #                        and process it through the virtual_alias_maps.
    #                        List all domains you want to manage aliases for, separated by spaces.
    virtual_alias_domains = domain1.com domain2.net domain3.org

    # virtual_alias_maps: Specifies the file that contains your alias mappings.
    #                     'hash:' indicates that Postfix will use a DBM database
    #                     derived from the plain text file.
    virtual_alias_maps = hash:/etc/postfix/virtual

    # Optional: Ensure no virtual mailbox related settings are active if you are
    #           not using local mailboxes for virtual users.
    # # virtual_mailbox_domains =
    # # virtual_mailbox_maps =
    # # virtual_mailbox_base =
    # # virtual_uid_maps =
    # # virtual_gid_maps =

Save the file and exit the editor.

Step 2: Create the Virtual Alias Map File (/etc/postfix/virtual)

This file contains the specific rules for how email addresses within your virtual_alias_domains should be handled.

Create and open this file for editing:

sudo nano /etc/postfix/virtual

Add your alias entries. The format is original_email_address followed by one or more destination_address(es), separated by whitespace.

    # Format: virtual_email_address      destination_address(es)

    # --- Aliases for domain1.com ---
    info@domain1.com        your_local_system_user_one   # Deliver to a local UNIX user (e.g., 'ubuntu', 'admin')
    sales@domain1.com       another_local_system_user    # Deliver to a different local UNIX user
    support@domain1.com     external_email@outlook.com   # Forward to an external email address
    webmaster@domain1.com   root, other_admin@external.com # Forward to multiple destinations (comma-separated)
    contact@domain1.com     info@domain1.com             # Forward to another virtual alias within Postfix

    # --- Aliases for domain2.net ---
    hello@domain2.net       your_local_system_user_one   # Can go to the same local system user
    admin@domain2.net       external_admin@another-company.com # Forward externally
    feedback@domain2.net    /dev/null                    # Silently discard incoming mail for this address
    press@domain2.net       contact@domain1.com          # Forward to an alias in another virtual domain

    # --- Catch-all aliases (use with caution, can attract a lot of spam) ---
    # Any email to domain1.com not explicitly listed above will go to catchall_for_d1@example.com
    @domain1.com            catchall_for_d1@example.com

    # Any email to domain2.net not explicitly listed above will go to some_local_user_for_d2
    @domain2.net            some_local_user_for_d2

Destinations can be: * Local system user: A valid username on your server (e.g., root, ubuntu). Mail will be delivered to /var/mail/username. * External email address: A full email address outside your server. * Another virtual alias: An address defined elsewhere in this same virtual file. /dev/null: To discard incoming mail silently without bounce. Multiple destinations: Separate multiple destinations with a comma and space.

Save the file and exit.

Step 3: Apply Changes and Reload Postfix

After making changes to the virtual file, you must convert it into a database format that Postfix can quickly look up. Then, reload the Postfix service to apply the new configuration.

sudo postmap /etc/postfix/virtual sudo systemctl reload postfix

Step 4: Configure DNS MX Records for Each Domain

This is a critical step that happens outside your server, at your domain registrar or DNS hosting provider. For other mail servers on the internet to know where to send mail for your domains, you must set up MX (Mail Exchanger) records.

For each of your domains (domain1.com, domain2.net, domain3.org), add an MX record pointing to your server's FQDN (the value you set in myhostname in main.cf).

Example DNS Configuration (at your domain registrar's panel):

# DNS for domain1.com:
# Type  Name   Value                            Priority
MX    @      mail.your_server_fqdn.com.       10  # This tells mail servers to send mail to mail.your_server_fqdn.com
A     @      YOUR_SERVER_PUBLIC_IP            # A record for the main domain
A     www    YOUR_SERVER_PUBLIC_IP            # A record for the www subdomain

# DNS for domain2.net:
# Type  Name   Value                            Priority
MX    @      mail.your_server_fqdn.com.       10
A     @      YOUR_SERVER_PUBLIC_IP
A     www    YOUR_SERVER_PUBLIC_IP

# DNS for domain3.org:
# Type  Name   Value                            Priority
MX    @      mail.your_server_fqdn.com.       10
A     @      YOUR_SERVER_PUBLIC_IP
A     www    YOUR_SERVER_PUBLIC_IP

# Also, ensure your mail server's hostname resolves to your server's IP:
A     mail   YOUR_SERVER_PUBLIC_IP