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Configuring Postfix as a Gmail Relay on CentOS

This tutorial should work on any distro based on RedHat, but I have only tested it on CentOS 6.4.
You will need to run the commands as root.

I found a ton of how-tos and tutorials on how to set up Postfix as a Gmail relay, but most of them required making a client certificate or were incomplete. After fighting with getting Postfix set up on CentOS 6.1, and browsing the internet for many, many days, I finally got it working. Part of the configuration is based on the pages found here and here.

Installing Postfix

Installing Postfix is easy, just run this command as root:

yum install postfix mailx cyrus-sasl-plain
Thanks to Jonathan for pointing that out.

Configuring

Basically, you need to create a password file so that Postfix can authenticate to Gmail's servers. You do this by creating a file named sasl_passwd in /etc/postfix. Replace smtp_user and smtp_passwd with their respective values.

echo "smtp.gmail.com    smtp_user:smtp_passwd" > /etc/postfix/sasl_passwd
You then hash that file so that the password is not stored in clear text. This command will create a file named sasl_passwd.db in the /etc/postfix/ directory.

postmap hash:/etc/postfix/sasl_passwd
After that is done, add these to the bottom of /etc/postfix/main.cf. This is assuming that your root certificates installed from openssl are located in /etc/pki/tls/certs/ca-bundle.crt.

smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
# Secure channel TLS with exact nexthop name match.
smtp_tls_security_level = secure
smtp_tls_mandatory_protocols = TLSv1
smtp_tls_mandatory_ciphers = high
smtp_tls_secure_cert_match = nexthop
smtp_tls_CAfile = /etc/pki/tls/certs/ca-bundle.crt
relayhost = smtp.gmail.com:587
After that is done, restart postfix:

service postfix restart

 

Now test it to make sure it is working. Run this:

mail email@domain<br< a="">>Fill in the subject, put something in the body and then type a . and hit enter.