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.
Start by installing Postfix and mailutils.
sudo apt install postfix mailutils
Assuming that your external SMTP server listens to
SMTP.DOMAIN.TLD:587, replace the contents of
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
Create credentials lookup table
/etc/postfix/sasl_passwd and insert your login
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
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