Postfix Configuration to use Gmail SMTP in Ubuntu

First you have to enable the access for less secure apps option in gmail settings.

1. Update Ubuntu and install required packages.......

      $ sudo apt-get update
      $ sudo apt-get install postfix mailutils libsasl2-2 ca-certificates libsasl2-modules 

2. Now configure the postfix 

     $ vi /etc/postfix/main.cf
             relayhost = [smtp.gmail.com]: 587
             smtp_use_tls = yes
             smtp_sasl_auth_enable = yes
             smtp_sasl_password_maps = hash:/etc/postfix/sasl/sasl_passwd
             smtp_sasl_security_options = noanonymous
             smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt

3. Now specify the Gmail username and password
     
      $ vi /etc/postfix/sasl/sasl_passwd
                      [smtp.gmail.com]: 587                 username@gmail.com:password

4. Now give the permission to sasl_passwd file and update postfix to use sasl_passwd file:
     
     $ sudo chmod 400 /etc/postfix/sasl/sasl_passwd
     $ sudo postmap /etc/postfix/sasl/sasl_passwd

5. Now validate certificate 

     $ cat /etc/ssl/certs/thawte_Primary_Root_CA.pem | sudo tee -a /etc/postfix/sasl/ca-certificates.crt
     $ sudo /etc/init.d/postfix reload

6. Now test the configurations by sending a mail
   
    $ echo "Test mail" | mail -s "Test Mail" username@domain.com

       Check the mail in receiver's inbox OR sender's sent box. For troubleshooting check the log:

   $ sudo tail -f /var/log/mail.log                                      //Try to find the cause of error.//