Relaying Postfix SMTP via Gmail

Install the required packages

sudo apt-get install ca-certificates
sudo aptitude install postfix libsasl2 libsasl2-modules

Configure Postfix

sudo mkfifo /var/spool/postfix/public/pickup

You’ll want the following lines in your /etc/postfix/main.cf file:

relayhost = [smtp.gmail.com]:587

smtp_use_tls = yes

smtp_sasl_auth_enable = yes

smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd

smtp_sasl_security_options = noanonymous

smtp_tls_loglevel = 1

smtp_tls_per_site = hash:/etc/postfix/tls_per_site

smtp_tls_CAfile = /etc/ssl/certs/Equifax_Secure_CA.pem

smtp_tls_session_cache_database = btree:/var/lib/postfix/smtp_tls_session_cache

The above lines are telling Postfix that you want to relay mail through gmail on a specific port, telling it to authenticate, and where to find the username and password. 
The last three lines specify the authentication types supported, where the certificate authority file is and that it should use tls.
Define Username and Password

Next we’ll need to populate the sasl_passwd file. 

sudo nano /etc/postfix/sasl_passwd
Add the following content:

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

This file should have restrictive permissions and then needs to be translated into a .db that Postfix will read.

sudo chmod 400 /etc/postfix/sasl_passwd
sudo postmap /etc/postfix/sasl_passwd

At this point you can restart Postfix and it should work, however it will complain about not being able to authenticate the certificate. 
To take care of this issue we’ll use the ca-certificate package we installed and tell it where it can validate the certificate.


cat /etc/ssl/certs/Equifax_Secure_CA.pem | sudo tee -a /etc/postfix/cacert.pem

We also need to populate a TLS per site file.

sudo nano /etc/postfix/postmap tls_per_site

Add the following content:

smtp.gmail.com MUST

This file needs to be translated into a .db that Postfix will read.

sudo postmap /etc/postfix/tls_per_site

Go ahead and reload postfix and you should be set.

sudo /etc/init.d/postfix reload