How to configure Gmail as a Mail Relay with Postfix

How to configure Gmail as a Mail Relay with Postfix

Introduction

Postfix is a flexible mail server that is available on most Linux distribution. Though a full feature mail server, Postfix can also be used as a simple relay host to another mail server, or smart host. This tutorial will describe how to configure Postfix as a relay through Gmail.

Simple Authentication and Security Layer (SASL) is a standard authentication framework supported by many services including Postfix.

Requirements

• CentOS 7 or Red Hat Enterprise Linux 7
• Valid Gmail or Google App credentials

Install Packages

Make sure Postfix, the SASL authentication framework, and mailx are all installed.

yum -y install postfix cyrus-sasl-plain mailx

Postfix will need to be restarted before the SASL framework will be detected.

systemctl restart postfix

Postfix should also be set to start on boot.

systemctl enable postfix

Configure Postfix

Open the /etc/postfix/main.cf and add the following lines to the end of the file.

myhostname = hostname.example.com

relayhost = [smtp.gmail.com]:587
smtp_use_tls = yes
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_tls_CAfile = /etc/ssl/certs/ca-bundle.crt
smtp_sasl_security_options = noanonymous
smtp_sasl_tls_security_options = noanonymous

The myhostname parameter is optional. If the hostname is not specified, Postfix will use the fully-qualified domain name of the Linux server.

Save the main.cf file and close the editor.

Configure Postfix SASL Credentials

The Gmail credentials must now be added for authentication. Create a /etc/postfix/sasl_passwd file and add following line:

[smtp.gmail.com]:587 username:password

The username and password values must be replaced with valid Gmail credentials. The sasl_passwd file can now be saved and closed.

A Postfix lookup table must now be generated from the sasl_passwd text file by running the following command.

postmap /etc/postfix/sasl_passwd

Access to the sasl_passwd files should be restricted.

chown root:postfix /etc/postfix/sasl_passwd*
chmod 640 /etc/postfix/sasl_passwd*

Lastly, reload the Postfix configuration.

systemctl reload postfix

Test the Relay

Use the mail command to test the relay.

echo "This is a test." | mail -s "test message" [email protected]

The destination address should receive the test message.

Troubleshoot Delivery Issues

The maillog can be reviewed if the test message is not successfully delivered. Open another shell and run tail while performing another test.

tail -f /var/log/maillog

If there are not enough details in the maillog to determine the problem, then the debug level can be increased by adding the following lines to the /etc/postfix/main.cf.

debug_peer_list=smtp.gmail.com
debug_peer_level=3

The Postfix configuration must be reloaded after updating the main.cf file.

systemctl reload postfix

Remember to remove the debug settings when testing is complete. The verbose logs can have a negative impact on server performance.