Configure Postfix to Send Mail Using an External SMTP Server

Updated Thursday, October 31, 2019 by Linode Contributed by Santiago Ti

Note

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

There are many reasons why you would want to configure Postfix to send email using an external SMTP provider such as Mandrill, SendGrid, Amazon SES, or any other SMTP server. One reason is to avoid getting your mail flagged as spam if your current server’s IP has been added to a spam list.

In this tutorial, you will learn how to install and configure a Postfix server to send email through Mandrill, or SendGrid.

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.

Updated Guide for Gmail and Google Apps

We’ve got an updated version of this guide that works with Gmail’s new security features!

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

Prerequisites

Before starting this tutorial, you should have:

• Your fully qualified domain name (FQDN)

• All updates installed :

  sudo apt-get update
  

• A valid username and password for the SMTP mail provider, such as Mandrill, or SendGrid

• Make sure the libsasl2-modules package is installed and up to date:

  sudo apt-get install libsasl2-modules
  

Note

This guide is written for a non-root user. Commands that require elevated privileges are prefixed with sudo. If you’re not familiar with the sudo command, you can check our Users and Groups guide.

Installing Postfix

In this section, you will install Postfix and set the domain and hostname.

1. Install Postfix with the following command:

   sudo apt-get install postfix
   

2. During the installation, a prompt will appear asking for your General type of mail configuration.

   

   Select Internet Site.

3. Enter the fully qualified name of your domain, fqdn.example.com.

   

4. Once the installation is finished, open the /etc/postfix/main.cf file with your favorite text editor:

   sudo nano /etc/postfix/main.cf
   

5. Make sure that the myhostname parameter is configured with your server’s FQDN:

   /etc/postfix/main.cf

   1	myhostname = fqdn.example.com

Configuring SMTP Usernames and Passwords

Usernames and passwords are generally stored in a file called sasl_passwd in the /etc/postfix/ directory. In this section, you’ll add your external mail provider credentials to this file and to Postfix.

If you want to use Mandrill, or SendGrid as your SMTP provider, you may want to reference the appropriate example while working on this section. For Google Apps and Gmail-specific settings, check out our Configure Postfix to Send Mail Using Gmail and Google Apps on Debian or Ubuntu guide.

1. Open or create the /etc/postfix/sasl_passwd file, using your favorite text editor:

   sudo nano /etc/postfix/sasl_passwd
   

2. Add your destination (SMTP Host), username, and password in the following format:

   /etc/postfix/sasl_passwd

   1	[mail.isp.example] username:password

   Note

   If you want to specify a non-default TCP Port (such as 587), then use the following format:

   /etc/postfix/sasl_passwd

   1	[mail.isp.example]:587 username:password

3. Create the hash db file for Postfix by running the postmap command:

   sudo postmap /etc/postfix/sasl_passwd
   

If all went well, you should have a new file named sasl_passwd.db in the /etc/postfix/ directory.

Securing Your Password and Hash Database Files

The /etc/postfix/sasl_passwd and the /etc/postfix/sasl_passwd.db files created in the previous steps contain your SMTP credentials in plain text.

For security reasons, you should change their permissions so that only the root user can read or write to the file. Run the following commands to change the ownership to root and update the permissions for the two files:

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

Configuring the Relay Server

In this section, you will configure the /etc/postfix/main.cf file to use the external SMTP server.

1. Open the /etc/postfix/main.cf file with your favorite text editor:

   sudo nano /etc/postfix/main.cf
   

2. Update the relayhost parameter to show your external SMTP relay host. Important: If you specified a non-default TCP port in the sasl_passwd file, then you must use the same port when configuring the relayhost parameter.

   /etc/postfix/main.cf

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

   Note

   Check the appropriate Google Apps, Mandrill, or SendGrid section for the details to enter here.

3. At the end of the file, add the following parameters to enable authentication:

   /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. Save your changes.

5. Restart Postfix:

   sudo service postfix restart
   

Testing Postfix

The fastest way to test your configuration is to send an email to any unrelated email address, using the mail command:

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

You may have to install mailutils to use the mail command:

sudo apt-get install mailutils

Alternatively, you can use Postfix’s own sendmail implementation, by entering lines similar to those shown below:

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

Examples of Postfix Configurations with Different Providers

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.

Settings for Mandrill

Use these settings for Mandrill.

1. For /etc/postfix/sasl_passwd, use the following configuration with your own credentials:

   /etc/postfix/sasl_passwd

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

2. For /etc/postfix/main.cf, use the following relayhost:

   /etc/postfix/main.cf

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

3. Create the hash db file for Postfix by running the postmap command:

   sudo postmap /etc/postfix/sasl_passwd
   

4. Restart Postfix:

   sudo service postfix restart
   

Settings for SendGrid

Use these settings for SendGrid.

1. For /etc/postfix/sasl_passwd, use the following configuration with your own credentials:

   /etc/postfix/sasl_passwd

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

2. For /etc/postfix/main.cf, use the following relayhost:

   /etc/postfix/main.cf

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

3. Create the hash db file for Postfix by running the postmap command:

   sudo postmap /etc/postfix/sasl_passwd
   

4. Restart Postfix:

   sudo service postfix restart
   

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This guide is published under a CC BY-ND 4.0 license.