In this tutorial, we will walk through the process of assigning unique IP addresses for outgoing mail in a Postfix mail server. This configuration is particularly useful if you are running a mail service and need to manage different users who send emails from your server.

Why Configure Per-User Ips?

If you host a mail server and one of your users is sending large amounts of spam, either because their account was compromised or their website scripts were infected, it can lead to your server’s IP address being blacklisted.

When this happens, all users sharing the same server IP might experience issues with email delivery, even if they are following good practices. By configuring individual IPs for each user, you can prevent one bad user from affecting others, as their IP can be isolated. This also allows you to temporarily disable the email sending for a specific user if needed.

Before you begin

Ensure that your server has multiple IP addresses configured on its network interface. You will use these IPs to assign to each user account for sending outgoing mail.

Step-by-Step Configuration

1. Create a Transport Map File

The first step is to create a file named sdd_transport_maps.regexp in the /etc/postfix/ directory. This file will map users or domains to their respective mail transport methods.

Here is an example format for this file:

/@user1-domain\.tld$/       user1:
@user2-domain\.tld$/       user2:
@user3-domain\.tld$/       user3:

• Replace user1-domain\.tld with the actual domain name associated with each user. For example, replace user1-domain\.tld with yourdomain\.com for a user on your domain.
• Replace user1, user2, and user3 with the actual usernames of your mail accounts.

This setup will ensure that emails sent from users with the domains listed will be handled by the specified users.

2. Configure Transport for Each User

Next, open the /etc/postfix/master.cf file and add specific transport entries for each user. This defines how mail will be routed for each user and specifies which IP address will be used for outgoing mail.

Add the following entries for each user:

user1  unix -       -       n       -       -       smtp
   -o smtp_bind_address=10.10.10.1
   -o smtp_helo_name=user1-domain.tld
   -o syslog_name=postfix-user1

user2  unix -       -       n       -       -       smtp
   -o smtp_bind_address=10.10.10.2
   -o smtp_helo_name=user2-domain.tld
   -o syslog_name=postfix-user2

user3  unix -       -       n       -       -       smtp
   -o smtp_bind_address=10.10.10.3
   -o smtp_helo_name=user3-domain.tld
   -o syslog_name=postfix-user3

• Replace smtp_bind_address=10.10.10.x with the actual IP address dedicated to each user.
• Set the smtp_helo_name to the user’s domain, which is used to identify the sending mail server in the email headers.
• The syslog_name setting ensures that log entries are clearly identified for each user.

For example, if you have a user with the domain mysite.com, the configuration would look like this:

mysite  unix -       -       n       -       -       smtp
   -o smtp_bind_address=10.10.10.1
   -o smtp_helo_name=mysite.com
   -o syslog_name=postfix-mysite

3. Update Main Configuration File

Now, you need to tell Postfix to use the transport map file. Open the /etc/postfix/main.cf file and add the following line:

sender_dependent_default_transport_maps = regexp:/etc/postfix/sdd_transport_maps.regexp

This setting ensures that Postfix will use the transport map you created earlier to assign specific IP addresses for each user’s outgoing mail.

4. Restart Postfix

After making the necessary changes, restart the Postfix service to apply the configuration:

service postfix restart

That’s It!

Once you’ve completed these steps, your Postfix mail server will be configured to send mail from different IPs depending on the user. This approach ensures that if one user’s IP gets blacklisted, the other users will not be affected. You can also disable mail sending for a specific user without impacting others.

Test the configuration by sending an email from each user account to verify that the mail is being sent from the correct IP address.