Configure Postfix to Send Email Using External SMTP Servers

Note

This guide was originally written for Debian 7. It has since been tested to work with Debian 9.

You may want to configure Postfix to use an external SMTP provider like Mandrill, and SendGrid so you no longer have to build, maintain, and scale your own SMTP relay server. Another reason is to avoid getting your mail flagged as spam if your current server’s IP has been added to a spam list. This guide shows you how to configure Postfix to use an external SMTP provider and also shows specific examples for Mandrill, and SendGrid. However, you can apply the steps in this guide to configure Postfix to use any external SMTP provider.

Note About Email at Linode

This guide may involve or result in sending email. In an effort to fight spam, Linode restricts outbound connections on ports 25, 465, and 587 on all Linodes for new accounts created after November 5th, 2019. For more information, please see Sending Email on Linode.

Postfix allows you to route and deliver emails and uses the Simple Mail Transfer Protocol (SMTP).

The majority of the Postfix configuration process is completed in the main.cf and master.cf files that are located in the /etc/postfix/ directory. The /etc/postfix directory is available once you have installed Postfix on your Linux system.

At a high-level Postfix configuration involves the following steps:

1. Gather prerequisites, which include a fully qualified domain name(FQDN), updating your system, and installing the libsasl2-module package
2. Install Postfix on your system
3. Configure SMTP usernames and passwords
4. Secure your password and hash database files
5. Configure the relay server
6. Test your Postfix configuration
7. Set up Postfix with Mandrill, and SendGrid

Note

If you’re using Gmail or Google Workspace, see our Configure Postfix to Send Mail Using Gmail and Google Workspace on Debian or Ubuntu guide instead.

In this section, you complete all prerequisite steps to configure Postfix to use an external SMTP server. Prerequisites to configure Postfix using an external SMTP server are:

1. Ensure you have fully qualified domain name (FQDN). The Fully Qualified Domain Name (FQDN) is the absolute domain name, including subdomains, top-level domain, and root zone, that will direct queries under the Domain Name System (DNS) to an exact location—in this context, to your Linode. For a detailed explanation, see What is FQDN For? Linode Community Q&A.

2. Ensure your system is up-to-date:

    apt-get update && apt-get upgrade
   

3. Install the libsasl2-modules package:

    sudo apt-get install libsasl2-modules
   

Your system is now ready to install Postfix.

1. Install Postfix by running the following command:

    sudo apt-get install postfix
   

2. You receive a prompt asking for your General type of mail configuration. Select Internet Site from the options.

1. Enter your fully qualified domain name when asked for your System mail name. An example FQDN is fqdn.example.com.

1. Once the installation is complete, open the /etc/postfix/main.cf file using your preferred text editor. Edit the file to add your Linode’s FQDN to the myhostname configuration, if it is not already configured, and save your changes.

   File: /etc/postfix/main.cf

   1	myhostname = fqdn.example.com

Usernames and passwords are stored in the /etc/postfix/sasl_passwd file. In this section, you add your external mail provider credentials to the sasl_passwd Postfix configuration file.

Note

The examples in this section provide the general steps to configure Postfix to use an external SMTP provider. If you want to use Mandrill or SendGrid as your SMTP provider, you can refer to the examples in the Postfix Configuration with Mandrill, and SendGrid section of this guide.

First, open or create the /etc/postfix/sasl_passwd file:

1. Using a text editor of your choice, open the /etc/postfix/sasl_passwd file. If it does not yet exist, create it.

2. Add the example line to your sasl_passwd file and replace username and password with your SMTP provider credentials.

   File: /etc/postfix/sasl_passwd

   1	[mail.isp.example] username:password

3. Create a Hash database file for Postfix using the postmap command. This command creates a new file named sasl_passwd.db in the /etc/postfix/ directory.

    sudo postmap /etc/postfix/sasl_passwd
   

In the previous section you added plain text credentials to the /etc/postfix/sasl_passwd and /etc/postfix/sasl_passwd.db files. For this reason, you have to change the each file’s permissions to restrict access to all users other than the root user.

1. Run the following command to change your each file’s permissions:

    sudo chown root:root /etc/postfix/sasl_passwd /etc/postfix/sasl_passwd.db
    sudo chmod 0600 /etc/postfix/sasl_passwd /etc/postfix/sasl_passwd.db
   

You are now ready to provide the configurations needed by Postfix to use the external SMTP server. This configuration tells Postfix to deliver mail indirectly via a relay host, which in this case, is an external SMTP server.

Note

Refer to the Postfix Configuration with Mandrill, and SendGrid section of this guide for specific relay host configurations for Mandrill, and SendGrid.

1. Using a text editor, open the /etc/postfix/main.cf file.

2. Update the relayhost configuration with your external SMTP relay host. Replace mail.isp.example with your provider’s information. If you specified a non-default TCP port in the sasl_passwd file, then use the same port when configuring the relay host. The example uses 587 as its port number.

   File: /etc/postfix/main.cf

   1 2	# specify SMTP relay host relayhost = [mail.isp.example]:587

3. Add the end of the file, add the example file’s parameters to enable authentication and save the changes you made to your main.cf file.

   File: /etc/postfix/main.cf

   1 2 3 4 5 6 7 8 9 10

   # enable SASL authentication smtp_sasl_auth_enable = yes # disallow methods that allow anonymous authentication. smtp_sasl_security_options = noanonymous # where to find sasl_passwd smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd # Enable STARTTLS encryption smtp_use_tls = yes # where to find CA certificates smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt

4. Restart Postfix to enable your configurations:

    sudo service postfix restart
   

You can now test your Postfix Configurations by using your system’s mail utility.

1. Install the mailutils package to use the mail utility.

    sudo apt-get install mailutils
   

2. Compose an email to verify that your system is able to successfully send it. Replace the email address with your own and your intended recipient’s email address.

    echo  "body of your email" | mail -s "This is a subject" -a "From:you@example.com" recipient@elsewhere.com
   

   Note

   You can use Postfix’s own Sendmail implementation to test your Postfix configuration. Replace the example’s values with your own. When you are done drafting your email type ctrl d.

   sendmail recipient@elsewhere.com
   From: you@example.com
   Subject: Test mail
   This is a test email
   

This section shows you settings for some popular mail services you can use as external SMTP servers. You may have to do some fine-tuning on your own to avoid Postfix logins being flagged as suspicious.

1. Open your /etc/postfix/sasl_passwd file and replace USERNAME and API_KEY with your own Mandrill credentials and save your changes.

   File: /etc/postfix/sasl_passwd

   1	[smtp.mandrillapp.com]:587 USERNAME:API_KEY

2. Open your /etc/postfix/main.cf file and add the Mandrill relay host information included in the example file.

   File: /etc/postfix/main.cf

   1	relayhost = [smtp.mandrillapp.com]:587

3. Create a hash database file for Postfix using the postmap command:

   sudo postmap /etc/postfix/sasl_passwd
   

4. Restart Postfix to enable your new configurations:

   sudo service postfix restart
   

1. Open your /etc/postfix/sasl_passwd file and replace USERNAME and PASSWORD with your own SendGrid credentials and save your changes.

   File: /etc/postfix/sasl_passwd

   1	[smtp.sendgrid.net]:587 USERNAME:PASSWORD

2. Open your /etc/postfix/main.cf file and add the SendGrid relay host information included in the example file.

   File: /etc/postfix/main.cf

   1	relayhost = [smtp.sendgrid.net]:587

3. Create a hash database file for Postfix using the postmap command:

   sudo postmap /etc/postfix/sasl_passwd
   

4. Restart Postfix to enable your new configurations:

   sudo service postfix restart
   

All Postfix configuration files are stored in the /etc/postfix directory. You can use the postconf command to view all of your system’s Postfix configurations and their details. This is a great way to verify that your Postfix configuration values are as you expect them to be.

To view all the available postconf options, view its manual pages using the man postconf command.

Postfix configuration files are located in the /etc/postfix/ directory. Most of your configurations are made in either the main.cf or master.cf files. To learn more about Postfix configuration files see Postfix’s README.