First of all you need to install and configure Postfix to Use Gmail SMTP on Ubuntu.
Install all necessary packages:
$ sudo apt-get install postfix mailutils libsasl2-2 ca-certificates libsasl2-modules
If you do not have postfix installed before, postfix configuration wizard will ask you some questions. Just select your server as Internet Site and for FQDN use something like mail.example.com
Then open your postfix config file:
$ sudo gedit /etc/postfix/main.cf
and add following lines to it:
relayhost = [smtp.gmail.com]:587
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_tls_CAfile = /etc/postfix/cacert.pem
smtp_use_tls = yes
You might have noticed that we haven’t specified our Gmail username and password in above lines. They will go into a different file. Open/Create:
$ sudo /etc/postfix/sasl_passwd
And add following line:
[smtp.gmail.com]:587 USERMAIL@gmail.com:PASSWORD
If you want to use your Google App’s domain, please replace @gmail.com with your @domain.com.
Fix permission and update postfix config to use sasl_passwd file:
$ sudo chmod 400 /etc/postfix/sasl_passwd
$ sudo postmap /etc/postfix/sasl_passwd
Next, validate certificates to avoid running into error. Just run following command:
$ cat /etc/ssl/certs/Thawte_Premium_Server_CA.pem | sudo tee -a /etc/postfix/cacert.pem
Finally, reload postfix config for changes to take effect:
$ sudo /etc/init.d/postfix reload
sudo cp /usr/share/ca-certificates/mozilla/*.* /usr/lib/ICAClient/keystore/cacerts/
sudo cp /usr/share/ca-certificates/mozilla/*.* /etc/ssl/certs/
** You have to go into Gmail's settings for this account and allow older apps to send mail. Gmail blocks these by default