There are certain cases where you want to set up Postfix to send emails through an external server. One such case is when you are developing a website on your computer and want to test a contact form before uploading to your production server.

Sending e-mails from a dynamic IP is generally a bad idea, since most email providers will automatically flag the message as spam. That leaves us with the option of forwarding the message to a “real” SMTP server.

This tutorial will show you how to set up Postfix to relay emails through an external SMTP server. If you don’t have your own SMTP server, you can alternatively use a SMTP server from a free service, such as Gmail.

The steps below have been tested on Ubuntu.

Install Postfix

Start by installing Postfix and mailutils.

sudo apt install postfix mailutils

Configure Postfix

Assuming that your external SMTP server listens to SMTP.DOMAIN.TLD:587, replace the contents of /etc/postfix/main.cf with:

inet_interfaces = loopback-only
relayhost = [SMTP.DOMAIN.TLD]:587
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_use_tls = yes
smtpd_client_restrictions =
   permit_mynetworks
   reject

Create credentials lookup table

Open /etc/postfix/sasl_passwd and insert your login USERNAME and PASSWORD.

[SMTP.DOMAIN.TLD]:587 USERNAME:PASSWORD

Change the file permissions, so that only root has access to it.

sudo chown root:root /etc/postfix/sasl_passwd && sudo chmod 400 /etc/postfix/sasl_passwd

Convert the text file to a database file.

sudo postmap /etc/postfix/sasl_passwd

Restart Postfix

Finally, restart Postfix for changes to take effect.

sudo service postfix restart

That’s it! In case of troubleshooting, don’t forget to check the Postfix log file at /var/log/mail.log.